Object lens for optical pickup device, optical pickup device and optical information recording/reproducing device

ABSTRACT

An objective lens used for an optical pickup device wherein the optical pickup device includes: light source; and a converging optical system including the objective lens for converging a light beam emitted from the light source to an information recording surface of an optical information recording medium, and the optical pickup device is capable of recording and/or reproducing information by converging the light beam emitted from the light source to the information recording surface of the optical information recording medium with the converging optical system, the objective lens being a plastic single lens and satisfying following formulas when NA is an image-side numerical aperture required for recording and/or reproducing information to the optical information recording medium and f (mm) is a focal length of the objective lens. Even in a plastic single lens having a high NA, thermal aberration does not increase excessively, and in a plastic single lens of a refraction type, thermal aberration within the temperature range of practical use in an optical pickup device is suppressed within an allowable range.

TECHNICAL FIELD

The present invention relates to an optical pickup device, an opticalinformation recording/reproducing apparatus and an objective lens usedfor them, and particularly, relates to an optical pickup device, anoptical information recording/reproducing apparatus which are capable ofhigh density optical information recording or reproducing, and anobjective lens used for them.

BACKGROUND ART

Hitherto, a plastic single lens has been generally used as an objectivelens used in an optical pickup device or optical informationrecording/reproducing apparatus for recording or reproducing an opticalinformation recording medium such as a CD, MD and DVD.

Because of lower specific density than a glass lens, a plastic lens hasan advantage that it is possible to reduce the burden of an actuatordriving the objective lens for focusing and tracking, and to performtracking of the objective lens in this regard at high speed.

Also, a plastic lens produced by injection molding in a mold can bemass-produced by manufacturing a desired mold with high accuracy.Thereby, although it is made possible to exert high performance of thelens stably, it is made possible to plan to reduce the cost.

By the way, in recent years, study/development of new high-densityoptical disk system in which a blue-violet laser diode light sourcehaving a wavelength of approximately 400 nm and an objective lens havinga numerical aperture (NA) enhanced up to approximately 0.85 are used hasbeen progressed. By way of example, as for an optical disk performinginformation recording/reproducing with descriptions of an NA of 0.85 anda light source wavelength of 405 nm (hereinafter, such an optical diskis referred to as “high-density DVD”), it is possible to recordinformation of 20 to 30 GB per side on an optical disk having a diameterof 12 cm that is the same size as a DVD (an NA of 0.6, a light sourcewavelength of 650 nm and a storage capacity of 4.7 GB).

Here, in an optical pickup device for such a high-density DVD, sphericalaberration generated by refractive index change accompanying temperaturechange (hereinafter, such spherical aberration is referred to as“thermal aberration”) becomes a problem in case that an objective lenshaving a high NA is a plastic lens. Such a problem occurs owing to aplastic lens two orders of magnitude larger than a glass lens in termsof change of the refractive index. Usable temperature range becomes verynarrow in case that the objective lens having an NA of 0.85 used for ahigh-density DVD is a plastic lens because the thermal aberration isproportional to 4th power of the NA, and accordingly it becomes aproblem in practical use.

In JP Tokukaihei-11-337818A, an art of correcting such thermalaberration of a plastic single lens by using the diffraction effect of aring-shaped phase structure formed on its optical surface is described.

For correcting thermal aberration of a plastic lens having an NA of 0.85by this art, it is necessary to set a tilt of a spherical aberrationcurve in change of wavelength (hereinafter, such tilt of a sphericalaberration curve is referred to as “chromatic spherical aberration”)large. Therefore, it is impossible to use a laser diode having anemission wavelength that deviates from a standard wavelength by amanufacturing error, and selection of laser diodes becomes necessary,which causes a high cost.

A specific example with numerical values is shown below. An objectivelens whose lens data is shown in Table 1 is a plastic single lens havingan incident light beam diameter of 3 mm, a focal length of 2.5 mm, an NAof 0.6, a design wavelength of 650 nm and a design temperature of 25°C., and corrects thermal aberration by the diffraction effect of aring-shaped phase structure formed on the first surface (optical surfaceof a light source example). On the other hand, an objective lens whoselens data is shown in Table 2 is a plastic single lens having anincident light beam diameter of 3 mm, a focal length of 1.76 mm, an NAof 0.85, a design wavelength of 405 nm and a design temperature of 25°C., and corrects thermal aberration by the diffraction effect of aring-shaped phase structure formed on the first surface in the same wayas the objective lens of Table 1. Note that a power-of-ten number (e.g.2.5×10⁻³) is expressed by using E (e.g. 2.5×E−3) hereinafter (includinglens data in Tables). TABLE 1 Surface No. r(mm) d(mm) N650 νd Remarks 0∞ Light source 1   1.6603 2.0500 1.54090 56.7 Objective 2 −4.5237 1.0105lens 3 ∞ 0.6000 1.57756 30.0 Protective 4 ∞ layer Aspherical surfacecoefficients 1st surface 2nd surface κ −6.8755E−01   −7.9005E+00 A43.0995E−03   4.3885E−02 A6 2.6042E−04 −3.2001E−02 A8 4.5653E−05  1.1954E−02 A10 −1.2223E−04   −1.9590E−03 Diffraction surfacecoefficients 1st surface b2 −2.3969E−03 b4 −7.8946E−04

TABLE 2 Surface No. r(mm) d(mm) N405 νd Remarks 0 ∞ Light source 1  1.2099 2.4500 1.56013 56.7 Objective 2 −1.5783 0.3771 lens 3 ∞ 0.10001.61950 30.0 Protective 4 ∞ layer Aspherical surface coefficients 1stsurface 2nd surface κ −7.1214E−01 −4.3724E+01 A4   5.4718E−03  5.2395E−01 A6   5.1672E−03 −1.1813E+00 A8   1.5578E−03   1.2111E+00A10   1.0499E−03 −5.0156E−01 A12 −7.7777E−04   6.2662E−04 A14−1.4455E−05 A16   1.7285E−04 A18 −2.2142E−05 A20 −1.2407E−05 Diffractionsurface coefficients 1st surface b2 −7.6944E−03 b4 −8.9900E−03 b6  1.1465E−03 b8   2.2677E−04 b10 −3.3067E−04

An aspherical surface in such an objective lens is expressed by thefollowing Formula 1 when the optical axis direction is x-axis, theheight of the direction perpendicular to the optical axis is h and thecurvature radius of the optical surface is r. Note that κ is a constantof the cone and A_(2i) is an aspherical surface coefficient.$\begin{matrix}{{X = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + \kappa} \right){h^{2}/r^{2}}}}} + {\sum\limits_{i = 2}\quad{A_{2i}h^{2i}}}}}\quad} & {{Formula}\quad 1}\end{matrix}$

Additionally, in such an objective lens, the ring-shaped phase structureas a diffractive structure formed on the optical surface is expressed byan optical path difference added to a transmitted wave front by thediffractive structure. As for the optical path difference is expressedby the optical path difference function Φ_(b) (mm) defined by thefollowing Formula 2, when the height of the direction perpendicular tothe optical axis is h and b_(2i) are the diffractive surfacecoefficients (also referred to as optical path difference functioncoefficients). $\begin{matrix}{\Phi_{b} = {\sum\limits_{i = 1}\quad{b_{2i}h^{2i}}}} & {{Formula}\quad 2}\end{matrix}$

When a diffraction structure is formed on the optical surface of anobjective lens based on a value of the optical path difference functionΦ_(b) (mm), a ring surface is formed each time a value of the opticalpath difference function Φ_(b) (mm) is changed by n-times apredetermined wavelength λB (n is only a natural number). In the presentdescription, “the diffraction structure is optimized at a wavelength λBand a diffraction order n” indicates that a diffraction structure isdetermined in this way, and the wavelength is referred to as anoptimized wavelength or production wavelength.

Table 3 shows RMS values of thermal aberration when an ambienttemperature of the two objective lens has risen by 30° C., and RMSvalues of chromatic spherical aberration when incident wavelengthbecomes 5 nm longer than the design wavelength. TABLE 3 Thermalaberration Chromatic spherical (+30° C.) aberration(+5 nm) NA 0.6 0.010λrms 0.003 λrms NA 0.85 0.014 λrms 0.057 λrms

As found from Table 3, as for an objective lens having an NA of 0.6 hasa chromatic spherical aberration suppressed at 0.003 λrms even when thethermal aberration is corrected to 0.010 λrms, and accordingly a laserdiode having a wavelength deviating by 5 nm may be used. At the sametime, as for an objective lens having an NA of 0.85, the chromaticspherical aberration becomes 0.057 λrms when the thermal aberration iscorrected to 0.014 λrms as much as the objective lens having an NA of0.6, and accordingly a laser diode having a wavelength deviating by 5 nmcannot be used. Laser diodes used as a light source in an optical pickupdevice have variation of about ±5 nm in its emission wavelength, andaccordingly, selection of laser diodes becomes necessary and theproduction cost of the optical pickup device rises in case of theobjective lens having an NA of 0.85.

Note that, in the objective lenses of Tables 1 and 2, both of the changerates of the refractive indexes accompanying the temperature rise aremade −9.0×10⁻⁵ and the change rates of the wavelength of incident lightaccompanying the temperature rise are respectively made +0.2 nm/° C. and+0.05 nm/° C.

Also, in the lens data of Table 1, r (mm) denotes a curvature radius, d(mm) denotes a surface distance, N650 denotes a refractive index at awavelength of 650 nm and νd denotes an Abbe number at the d-line, and inthe lens data of Table 2, r (mm) denotes a curvature radius, d (mm)denotes a surface distance, N405 denotes a refractive index at awavelength of 405 nm and νd denotes an Abbe number at the d-line.

Furthermore, longitudinal chromatic aberration generated in an objectivelens becomes a problem in case of using a blue-violet laser diodegenerating light with a short wavelength of about 400 nm as a lightsource like such an optical pickup device for a high-density DVD. In anoptical pickup device, chromatic aberration of the objective lens isconsidered not to be a problem because laser light emitted from a laserdiode has a single wavelength (single mode). However, actually aphenomenon referred to as mode hopping that a center wavelength isinstantly changed by several nm owing to temperature change, outputchange or the like, is caused. Because the mode hopping is a wavelengthchange caused instantly which a focusing mechanism cannot track, thereis caused a problem that a defocus component corresponding to movementof the image formation position is added and the converging ability ofthe objective lens is degraded when longitudinal chromatic aberration ofthe objective lens is corrected.

Because dispersion of general lens materials used for an objective lensis not so large within a range of 600 nm to 800 nm, which is thewavelength region of infrared laser diodes and red laser diodes, thedegradation of the converging ability of an objective lens due to modhopping did not become a problem in CDs and DVDs.

However, because dispersion of lens materials becomes very large in theregion of 400 nm, which is the wavelength region of blue-velvet laserdiodes, a wavelength change of even slightly several nm causes the imageformation position of the objective lens deviate largely. Therefore in ahigh-density DVD, the converging ability of an objective lens isdegraded largely and stable recording and reproducing might beimpossible when a laser diode light source causes mode hopping.

The present invention, which has been made in consideration ofcircumstances as described above, aims at providing a plastic singlelens that is applicable as an objective lens of an optical pickup deviceusing an objective lens having a high NA and has an availabletemperature range being sufficiently wide and slight degradation ofconverging ability owing to mode hopping of a light source.

Furthermore, the present invention aims at providing a plastic singlelens that is applicable as an objective lens of an optical pickup deviceusing an objective lens having a high NA, wherein it is possible to makeselection of laser diode light source unnecessary in the production stepof an optical pickup device without excessive increase of chromaticspherical aberration even when thermal aberration has been corrected inorder to extend the available temperature range.

Furthermore, the present invention aims at providing an optical pickupdevice where a plastic single lens of these is mounted, and an opticalinformation recording/reproducing apparatus where the optical pickupdevice is mounted.

DISCLOSURE OF INVENTION

An objective lens for an optical pickup device of claim 1 is anobjective lens used for an optical pickup device,

-   -   wherein the optical pickup device comprises: a light source; and        a converging optical system including the objective lens for        converging a light beam emitted from the light source to an        information recording surface of an optical information        recording medium, and the optical pickup device is capable of        recording and/or reproducing information by converging the light        beam emitted from the light source to the information recording        surface of the optical information recording medium with the        converging optical system, and    -   wherein the objective lens is a plastic single lens and        satisfies following formulas:        NA≧0.8  (1)        1.0>f>0.2  (2)    -   where NA is an image-side numerical aperture of the objective        lens, which is required for recording and/or reproducing        information to the optical information recording medium and        f (mm) is a focal length of the objective lens.

The variation of spherical aberration owing to change of the refractiveindex of the plastic single lens accompanying temperature rise (thermalaberration) increases in proportion to the focal length and 4th power ofthe NA. Accordingly, even in cases of increasing the NA for densifyingan optical information recording medium, it is made possible tocomparatively suppress the thermal aberration by reducing the focallength according thereto. Therefore, as for the objective lens of claim1, by setting the upper limit of the focal length as the formula (2),thermal aberration is prevented from increasing excessively even in caseof a plastic single lens having a high NA that satisfies the formula(1). Furthermore, as for a plastic single lens of a refraction type, itis impossible to make thermal aberration zero completely. However, it ispossible to suppress the thermal aberration in a temperature range ofpractical use of the optical pickup device into an allowable range bymaking the focal length not excess the upper limit of the formula (2).

On the other hand, though reduction of the focal length is advantageousfrom the viewpoint of suppressing the generation amount of thermalaberration, excessive reduction of the focal length is disadvantageousfrom the viewpoint of the working distance and image heightcharacteristics. As for design of an objective lens having a high NA,securing the focal length is a very important problem for preventingclash with an optical information recording medium. When the focallength is reduced excessively, the working distance is lost by thatamount, which is not favorable. When trying to obtain the same imageheight as an objective lens having a relatively long focal length,astigmatic aberration and coma aberration are degraded because anincident angle to an objective lens having a relatively short focallength increases. Accordingly, it is not favorable to reduce the focallength of the objective lens also from the viewpoint of image heightcharacteristics. Therefore, the objective lens of claim 1 secured thenecessary and sufficient working distance and image heightcharacteristics by setting the upper limit of the focal length as theformula (2).

An objective lens for the optical pickup device of claim 2 ischaracterized in that, in the invention of claim 1, when W(λ₀, T₀) is anRMS value of residual aberration of the objective lens a first ambienttemperature T₀=25° C. and W(λ₀, T₁) is an RMS value of residualaberration of the objective lens when light having the wavelength of λ₀(nm) which is a design wavelength thereof is incident to the objectivelens at the environmental temperature which is a second ambienttemperature T₁=55° C., ΔW defined byΔW=|W(λ₀ , T ₁)−W(λ₀ , T ₀)|  (3)satisfies a following formula:ΔW<0.035 λrms  (4)

For making a plastic single lens capable of be used within thetemperature range of practical use in an optical pickup device, it ispreferable to give temperature characteristics that make the focallength not excess the upper limit of the formula (2) and consequentlysatisfies the formula (4). Thereby, it is possible to perform goodrecording/reproducing of information for an optical informationrecording medium by using the plastic single lens within the temperaturerange of practical use in an optical pickup device.

An objective lens for the optical pickup device of claim 3 ischaracterized in that, in the objective lens for the optical pickupdevice of claim 1 or 2, the design wavelength λ₀ of the opticalobjective lens is not more than 500 nm, and in case that fB(λ₀, T₀) isan back focal length of the objective lens when light having awavelength of λ₀ (nm) is incident to the objective lens at anenvironmental temperature which is a first ambient temperature T₀=25° C.and fB(λ₁, T₀) is a back focal length of the objective lens when lighthaving a wavelength of λ₁ (nm) which is 5 nm longer than the wavelengthof λ₀ is incident to the objective lens in the environmental temperaturewhich is the first ambient temperature T₀=25° C., ΔfB defined byΔfB=|fB(λ₁ , T ₀)−fB(λ₀ , T ₀)|  (5)satisfies a following formula:ΔfB<0.001 mm  (6)

The longitudinal chromatic aberration due to mode hopping of a laserdiode increases in proportion to the focal length. Accordingly, even incase of using, for example, a blue-violet laser diode as the lightsource, it becomes possible to suppress the longitudinal chromaticaberration comparatively when the focal length is reduced correspondingthereto. As for a single lens of a refraction type, it is impossible tomake chromatic aberration zero completely. However, it is possible tosuppress the variation of wavefront aberration including a defocuscomponent less than 0.035 λrms for the variation of wavelength due tomode hopping of the blue-violet laser diode when variation of the backfocal length in increasing the incident wavelength by 5 nm is made lessthan 0.001 mm (the formula (6)) in an object lens, such as the objectivelens of claim 3, in which the focal length is set so as to satisfy theformula (2) and a blue-violet laser diode is used as a light source.Therefore, conversing ability is not degraded significantly even whenmode hopping is caused in switching from the reproducing condition tothe recording condition.

An objective lens for the optical pickup device of claim 4 ischaracterized in that, in the invention of any one of claims 1 to 3, theobjective lens is an objective lens of a finite conjugate type forconverging a diverging light beam emitted from the light source to theinformation recording surface of the optical information recordingmedium and satisfies a following formula:0.8>f>0.2  (6A)

The objective lens of claim 4 is preferable as an objective lens for anoptical pickup device of which miniaturization is required, and forexample, may be used as an objective lens for an optical pickup deviceinstalled in a portable optical disk player. In order to obtain anobjective lens of a finite conjugate type having an image formationmagnification of m and brightness as much as an objective lens of aninfinity type, it is necessary to design a lens having brightness (1-m)times as much as the image-side numerical aperture of the objective lensof an infinity type. The sign of m becomes minus and the substantialimage-side numerical aperture becomes larger than the image-sidenumerical aperture of the objective lens of an infinity type in casethat the objective lens is a finite conjugate type that converges adiverging light beam emitted from the light source on the informationrecording surface of the optical information recording medium.Accordingly, the thermal aberration is made large than the objectivelens of the infinity type when the objective lens of the finiteconjugate type is made a plastic single lens. In the objective lens ofclaim 4, by making the upper limit of the focal length further less thanthe formula (2) and setting it as the formula (6A), it is possible tosuppress thermal aberration in an allowable range of practical use evenin case of a plastic single lens of a finite conjugate type having ahigh NA as the NA satisfies the formula (1). In the objective lens ofthe finite conjugate type for converging a diverging light beam, theworking distance becomes larger as compared to the objective lens of theinfinity type having the same focal length. Accordingly, it is notdisadvantageous from the viewpoint of securing the working distance alsoin case of making the upper limit of the focal length further less thanthe formula (2) as the objective lens of claim 4.

An objective lens for an optical pickup device of claim 5 ischaracterized in that, in the invention of claim 4, m satisfiesfollowing formula when m is an image formation magnification of theobjective lens:0.2>|m|>0.02  (6B)

When the image formation magnification m is larger than the lower limitof the above-described formula (6B), even an objective lens having shortfocal length which satisfies the above-described formula (6A) can securea sufficient working distance. On the other hand, when the imageformation magnification m is smaller than the upper limit of the formula(6B), it is possible to suppress the thermal aberration within theallowable range of practical use because the substantial image-sidenumerical aperture does not increase excessively.

An objective lens used for an optical pickup device of claim 6 is anobjective lens used for an optical pickup device, wherein the opticalpickup device comprises a light source; and a converging optical systemincluding an objective lens for converging a light beam emitted from thelight source to an information recording surface of an opticalinformation recording medium, and the optical pickup device is capableof recording and/or reproducing information by converging the light beamemitted from the light source to the information recording surface ofthe optical information recording medium with the converging opticalsystem,

-   -   wherein the objective lens is a plastic single lens that        comprises a ring-shaped phase structure on at least one optical        surface, the ring-shaped phase structure comprising a plurality        of ring surfaces and formed so that adjacent ring surfaces        generate a predetermined optical path difference for incident        light, and satisfies following formulas:        NA≧0.8  (7)        1.3>f>0.2  (8)    -   where NA is an image-side numerical aperture of the objective        lens, which is required for recording and/or reproducing        information for the optical information recording medium and        f (mm) is a focal length of the objective lens.

In the plastic objective lens in which the numerical aperture NAsatisfies the formula (7), in case of correcting spherical aberration(thermal aberration) that is generated by the refractive index changeaccompanying temperature rise by an effect of the ring-shaped phasestructure formed on the optical surface thereof, the tilt (chromaticspherical aberration) of the spherical aberration curve in change of thewavelength becomes excessively large. Accordingly, it is impossible touse a laser diode having an emission wavelength that deviates from astandard wavelength by a manufacturing error, and selection of laserdiodes becomes necessary.

As described above, the variation of spherical aberration owing tochange of the refractive index of the plastic objective lens increasesin proportion to the focal length and 4th power of an NA. Accordingly,even in cases of the NA increasing for densifying an optical informationrecording medium, it is made possible to comparatively suppress thespherical aberration owing to the refractive index change of theobjective lens by reducing the focal length according thereto.

As for the objective lens of claim 6, it is possible to preventchromatic spherical aberration after correcting thermal aberration fromexcessively increasing, because the correction amount of thermalaberration due to the effect of the ring-shaped phase structure issuppress low by setting the upper limit of the focal length as theformula (8). As a result of this, as for an optical pickup device inwhich the objective lens due to the present invention is mounted, it ispossible to suppress the production cost because selection of laserdiodes in the production step. Meanwhile, though reduction of the focallength is advantageous from the viewpoint of suppressing the generationamount of thermal aberration, excessive reduction of the focal length isdisadvantageous from the viewpoint of the working distance and imageheight characteristics. Therefore, the objective lens of the presentinvention secured the necessary and sufficient working distance andimage height characteristics by setting the upper limit of the focallength as the formula (8).

In the present description, an objective lens indicates, in the narrowsense, a lens having the converging ability which is disposed at theposition closest to the optical information recording medium to faceopposite it in a state that an optical recording medium is loaded in theoptical pickup device, and in the broad sense, a lens capable of beingactuated with the lens at least in the optical axis direction by anactuator. Accordingly, in the present description, a numerical apertureof the objective lens on the optical information recording medium side(image side) indicates the numerical aperture of the lens surface of theobjective lens located closest to the optical information recordingmedium. Also in the present description, a necessary (predetermined)numerical aperture indicates a numerical aperture regulated by thestandard of respective optical information recording media or anumerical aperture of an objective lens having the diffraction limitability capable of obtaining a spot size required for recording orreproducing information depending on the wavelength of a used lightsource for respective optical information recording media.

Also in the present description, recording of information indicatesrecording information on an information recording surface of an opticalinformation recording medium like the above. In the present description,reproducing of information indicates reproducing information recorded onan information recording surface of an optical information recordingmedium like the above. An objective lens of the present invention may beused for only recording or only reproducing, and may be used for both ofrecording and reproducing. It may be used for recording for a certainoptical information recording medium and reproducing for another opticalinformation recording medium, or may be used for recording orreproducing for a certain optical information recording medium andrecording and reproducing for another optical information recordingmedium. The term reproducing here includes reading information simply.

As for an objective lens for the optical pickup device of claim 7, inthe invention of claim 6, the ring-shaped phase structure is adiffraction structure having a function for diffracting predeterminedincident light and the objective lens forms a converging wave frontwhich is converged on the information recording surface owing to aneffect obtained by combining a diffraction effect and a refractioneffect, and the above-described operation is exerted effectively andthereby it is preferable.

An objective lens for the optical pickup device of claim 8 has sphericalaberration characteristics that spherical aberration changes in anundercorrected direction when a wavelength of the incident light changesto a longer wavelength in the invention of claim 7.

Because a plastic single lens has a refractive index reduced bytemperature rise generally, spherical aberration changes in theovercorrected direction. Meanwhile, the emission wavelength of a laserdiode generally has a tendency to change in the increase direction bytemperature rise. Accordingly, by providing the objective lens havingthe above-described spherical aberration characteristics owing to theeffect of the diffraction structure, change of spherical aberration thatis made overcorrection by change of the refractive index accompanyingtemperature rise can be counterbalanced by change of sphericalaberration that is made undercorrection by change of the emissionwavelength of a laser diode due to temperature rise. As for even ahigh-NA plastic single lens, because the objective lens of the presentinvention has a focal length satisfying the formula (8), the correctionamount of thermal aberration owing to the effect of the diffractionstructure is small and chromatic spherical aberration after correctingthe thermal aberration does not become large excessively.

In the present description, an optical surface (diffraction surface) onwhich a diffraction structure is formed is a surface given an effect fordiffracting an incident light beam by providing relief for surface of anoptical element, e.g. a surface of a lens, and in case that there are aregion for generating diffraction and a region for not generatingdiffraction on the same optical surface, the region for generatingdiffraction. A diffraction structure or a diffraction pattern is theregion for generating diffraction. As the shape of the relief, forexample, a shape is formed on the optical element as substantiallyconcentric ring surfaces centered on the optical axis, and when itssection of the plane including the optical axis is seen, a serrate orstepwise shape is known as for respective ring surfaces, while theseshapes are included.

Furthermore, innumerable diffracted lights of 0th-order diffractedlight, +1st-order diffracted lights, ±2nd-order diffracted lights . . .are generated from the optical surface (diffraction surface) on which adiffraction structure is formed. For example, in case of a diffractionsurface having relief whose meridional section is serrate as above, theshape of the relief may be set such that the diffraction efficiency of aparticular order is made higher than diffraction efficiencies of theother orders, and in some cases, the diffraction efficiency of oneparticular order (e.g. +1st-order diffracted light) is made almost 100%.In the present invention, “a diffraction structure is optimized at awavelength of λ_(B) and a diffraction order of n” indicates setting theshape of the diffraction structure (relief) such that the diffractionefficiency of diffracted light of a diffraction order of n becomes 100%theoretically when light having a wavelength of λ_(B) is made incident.

An objective lens for the optical pickup device of claim 9 ischaracterized in that, in the invention of claim 7 or 8, when an opticalpath difference added to a wave front transmitted through thediffraction structure is denoted by an optical path difference functionΦ_(b) defined byΦ_(b) =b ₂ ·h ² +b ₄ ·h ⁴ +b ₆ ·h ⁶+ . . .(wherein b₂, b₄, b₆ . . . are 2nd-order, 4th-order, 6th-order . . .optical path difference function coefficients, respectively), afollowing formula is satisfied:−70<(b ₄ ·h _(max) ⁴)/(f·λ ₀·10⁻⁶·(NA·(1−m))⁴)<−20  (8A)wherein λ₀ (nm) is a design wavelength of the objective lens, h_(MAX) isan effective diameter maximum height (mm) of the optical surface onwhich the diffraction structure is formed and m is an image formationmagnification of the objective lens.

The objective lens for the optical pickup device of the presentinvention is preferably designed such that the 4th-order optical pathdifference function coefficient b₄, the effective diameter maximumheight h_(MAX) of the optical surface on which the diffraction structureis formed, the image formation magnification m, the focal length f andthe image-side numerical aperture NA satisfy the condition of theabove-described formula (8A). This condition is a condition forimproving the balance of the correction of thermal aberration and thegeneration amount of chromatic spherical aberration in a plastic lenswhere a diffraction structure is formed. In case of exceeding the lowerlimit of the above formula, the generation amount of chromatic sphericalaberration does not increase excessively because thermal aberration isnot overcorrected, and accordingly it is possible to use even a laserdiode having an emission wavelength that deviates from a standardwavelength by a manufacturing error, and it is possible to ease theselection condition of laser diodes to plan reduction of the costs.Meanwhile, in case of falling below the upper limit of the aboveformula, it is possible to provide a broad temperature range in which aplastic lens having a high NA can be used, because spherical aberrationgenerated by the refractive index change of the plastic lens having ahigh NA can be counterbalanced by spherical aberration generated bywavelength change of the laser diode.

As for an objective lens for the optical pickup device of claim 10, inthe invention of claim 6, the ring-shaped phase structure generates thepredetermined optical path difference for the incident light by formingthe adjacent ring surfaces so as to be displaced in an optical axisdirection each other, and the objective lens forms a converging wavefront which is converged on the information recording surface owing to arefraction effect, and the above-described operation is exertedeffectively and thereby it is preferable.

An objective lens for the optical pickup device of claim 11, in theinvention of claim 6, comprises at least one ring surface formed to bedisplaced to the inside compared with a ring surface adjacent to theside closer to the optical axis and at least one ring surface formed tobe displaced to the outside compared with a ring surface adjacent to theside closer to the optical axis, and the ring surface formed to bedisplaced to the inside compared with a ring surface adjacent to theside closer to the optical axis is formed closer to the optical axisthan the ring surface formed to be displaced to the outside comparedwith a ring surface adjacent to the side closer to the optical axis, andthermal aberration can be well corrected by configuring the ring-shapedphase structure in this way and thereby it is preferable.

An objective lens for the optical pickup device of claim 12 ischaracterized in that, in the invention of claim 10 or 11, a total ofthe ring surfaces is from 3 to 20.

An objective lens for the optical pickup device of claim 13 ischaracterized in that, in the invention of any one of claims 10 to 12,when Δ_(j) (μm) is a step amount of an arbitrary step of steps in theoptical axis direction at a boundary of mutually adjacent ring surfacesin a ring-shaped phase structure formed in a region from a height of 75%to a height of 100% of an effective diameter maximum height of theoptical surface on which the ring-shaped phase structure is formed and nis a refractive index of the objective lens at a design wavelength of λ₀(nm), m_(j) represented bym _(j) =INT(X)  (8B)(wherein X=Δ_(j)·(n−1)/(λ₀·10⁻³) and INT(X) is an integer obtained byhalf adjust of X) is an integer not less than 2.

In the objective lens of claims 10 and 11, a total of the ring surfacesis from 3 to 20, and additionally, when Δ_(j) (μ_(m)) is a step amountof an arbitrary step of steps in the optical axis direction at aboundary of mutually adjacent ring surfaces in a ring-shaped phasestructure formed in a region from a height of 75% to a height of 100% ofan effective diameter maximum height of the optical surface on which thering-shaped phase structure is formed and n is a refractive index of theobjective lens at a design wavelength of λ₀ (nm), m_(j) represented bythe above-described (8B) is an integer not less than 2, and mold processfor molding an objective lens becomes easy and time spent for the moldprocess can be reduced because it is possible to secure a large width ofa ring surface in the direction perpendicular to the optical axis.

Here, in case of the ring-shaped phase structure formed on the 1stsurface (the optical surface on the light source side), “formed to bedisplaced to the inside compared with a ring surface adjacent to theside closer to the optical axis” indicates “formed to be displaced inthe direction of the 2nd surface (the optical surface of the opticalinformation recording medium side) compared with a ring surface adjacentto the side closer to the optical axis”, and formed to be displaced tothe outside compared with a ring surface adjacent to the side closer tothe optical axis” indicates “formed to be displaced in the directionopposite to the direction of the 2nd surface (the optical surface of theoptical information recording medium side) compared with a ring surfaceadjacent to the side closer to the optical axis”. Also, in case of thering-shaped phase structure formed on the 2nd surface (the opticalsurface on the optical information recording medium side), “formed to bedisplaced to the inside compared with a ring surface adjacent to theside closer to the optical axis” indicates “formed to be displaced inthe direction of the 1st surface (the optical surface of the lightsource side) compared with a ring surface adjacent to the side closer tothe optical axis”, and formed to be displaced to the outside comparedwith a ring surface adjacent to the side closer to the optical axis”indicates “formed to be displaced in the direction opposite to thedirection of the 1st surface (the optical surface of the light sourceside) compared with a ring surface adjacent to the side closer to theoptical axis”.

An objective lens for the optical pickup device of claim 14 ischaracterized in that, in the invention of any one of claims 6 to 13,when W(λ₀, T₀) is an RMS value of residual aberration of the objectivelens when light having a wavelength of λ₀ (nm) which is a designwavelength thereof is incident to the objective lens at an environmentaltemperature which is a first ambient temperature T₀=25° C., W(λ₁, T₀) isan RMS value of residual aberration of the objective lens when lighthaving a wavelength of λ₁ (nm) which is 5 nm longer than the wavelengthof λ₀ is incident to the objective lens at the environmental temperaturewhich is the first ambient temperature T₀=25° C. and W(λ₂, T₁) is an RMSvalue of residual aberration of the objective lens when light having awavelength of λ₂ (nm) is incident to the objective lens at theenvironmental temperature which is a second ambient temperature T₁=55°C., ΔW1 and ΔW2 defined byΔW1=|W(λ₂ , T ₁)−W(λ₀ , T ₀)|  (9)ΔW2=|W(λ₁ , T ₀)−W(λ₀ , T ₀)|  (10)satisfy following formulas:ΔW1<0.035 λrms  (11)ΔW2<0.035 λrms  (12)wherein

-   when λ₀<600 nm, λ₂=λ₀+1.5 (nm) and-   when λ₀≧600 nm, λ₂=λ₀+6 (nm).

In a plastic lens having a high NA, when thermal aberration iscompletely corrected by the effect of the ring-shaped phase structureformed on the optical surface, chromatic spherical aberration increasesexcessively even in case that the focal length satisfies the formula (8)and it might be impossible to use a laser diode having an emissionwavelength that deviates from a standard wavelength, and accordingly itis necessary to balance the correction of thermal aberration and thegeneration amount of chromatic spherical aberration in lens design.Here, the formula (9) is a formula corresponding to thermal aberrationin case of temperature rising by 30° C., and the formula (10) is aformula corresponding to chromatic spherical aberration in case of thewavelength of incident light changing by 5 nm. As for the objective lensof the present invention, it is preferable that thermal aberration,chromatic spherical aberration and total aberration of the chromaticspherical aberration and the thermal aberration satisfy the formulas(11) and (12) and the after-described formula (13).

As the objective lens of claim 14, the condition that when λ₀<600 nm,λ₂=λ₀+1.5 (nm) corresponds the change (+0.05 nm/° C.) of the emissionwavelength owing to temperature rise of a blue-violet laser diode, andthe condition that when λ₀≧600 nm, λ₂=λ₀+6 (nm) corresponds the change(+0.2 nm/° C.) of the emission wavelength owing to temperature rise of ared laser diode.

In the present description, a design wavelength of an objective lens isthe wavelength making residual aberration of the objective lens theminimum in cases of making lights of various wavelength incident to theobjective lens on the same condition (the image formation magnification,temperature, incident light beam diameter and the like). Furthermore, inthe present description, a design temperature of an objective lens isthe temperature making residual aberration of the objective lens theminimum in cases of measuring the residual aberration of the objectivelens in various environmental temperatures on the same condition (theimage formation magnification, wavelength, incident light beam diameterand the like).

An objective lens for the optical pickup device of claim 15 ischaracterized in that the objective lens satisfies a following formulain the invention of claim 14, and thereby it is preferable.{square root}((ΔW1)²+(ΔW2)²)<0.05 λrms  (13)

An objective lens for the optical pickup device of claim 16 ischaracterized in that, in the invention of any one of claims 6 to 15,the objective lens is an objective lens of a finite conjugate type forconverging a diverging light beam emitted from the light source on theinformation recording surface and satisfies a following formula:1.1>f>0.2  (13A)The operation and effect of this invention is the same as the operationand effect of the invention of claim 4.

An objective lens for the optical pickup device of claim 17 ischaracterized in that, in the invention of claim 16, the objective lenssatisfies a following formula when m is an image formation magnificationof the objective lens:0.2>m|>0.02  (13B)The operation and effect of this invention is the same as the operationand effect of the invention of claim 5.

An objective lens for the optical pickup device of claim 18 ischaracterized in that, in the invention of any one of claims 1 to 17,the objective lens satisfies a following formula:0.8<d/f<1.8  (14)

-   -   where d (mm) is a lens thickness in an optical axis of the        objective lens and f (mm) is a focal length.

The formula (14) is a condition for securing good image heightcharacteristics, sufficient production tolerance and sufficient workingdistance in a high NA objective lens having a small diameter in whichthe focal length satisfies the formulas (2), (6A), (8) and (13A), andthere is an advantage that the 3rd-order astigmatic aberration componentin evaluating image height characteristics by wavefront aberration doesnot increase excessively and higher-order coma aberration componentsequal to or more than 5th-order does not increase excessively when avalue of d/f is larger than the lower limit of the formula (14).Meanwhile, there is an advantage that the 3rd-order spherical aberrationcomponent, 5th-order astigmatic aberration component, 3rd-order comaaberration component and astigmatic difference in evaluating imageheight characteristics by wavefront aberration do not increaseexcessively. Furthermore, because the gear radius of the optical surfaceon the light source side do not decrease excessively, it is possible tosuppress generation of coma aberration due to optical axis deviation ofoptical surfaces and to secure sufficient production tolerance. When avalue of d/f is larger than the lower limit of the formula (14), it ispossible to suppress generation of birefringence owing to moldingbecause the edge thickness is secured sufficiently and the uneventhickness ratio does not become excessively small, and meanwhile, when avalue of d/f is less than the upper limit of the formula (14), the lenscan be made light and driven by a smaller actuator and the workingdistance can be secured sufficiently because the lens thickness does notincrease excessively.

As for an objective lens for the optical pickup device of claim 19, inthe invention of any one of claims 1 to 18, the design wavelength of λ₀(nm) of the objective lens satisfies a following formula, and it ispossible to use it for an optical pickup device equipped with ashort-wavelength light source such as a blue-violet laser diode.500≧λ₀≧350  (15)

An objective lens for the optical pickup device of claim 20 ischaracterized in that, in the invention of any one of claims 1 to 19,the objective lens satisfies a following formula:0.40≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.63  (16)where

-   X1: a distance (mm) in an optical axis direction between a plane    that is perpendicular to an optical axis and tangent to a top of an    optical surface on a light source side and an optical surface on the    light source side in a most peripheral portion of an effective    diameter (position of the NA on a surface on the light source side    to which a marginal light beam is incident), wherein X1 is plus in a    case of measuring X1 in a direction of the optical information    recording medium with reference to the tangent plane, and minus in a    case of measuring X1 in a direction of the light source,-   X2: a distance (mm) in an optical axis direction between a plane    that is perpendicular to an optical axis and tangent to a top of an    optical surface on an optical information recording medium side and    an optical surface on the optical information recording medium side    in a most peripheral portion of an effective diameter (position of    the NA on a surface on the optical information recording medium side    to which a marginal light beam is incident), wherein X2 is plus in a    case of measuring X2 in a direction of the optical information    recording medium with reference to the tangent plane and minus in a    case of measuring X2 in a direction of the light source,-   N: a refractive index of the objective lens at the design wavelength    of λ₀,-   f: a focal length (mm) of the objective lens, and-   m: an image formation magnification of the objective lens.

Claim 20 regulates a conditional formula related to the sags of theoptical surface on the light source side and the optical surface on theoptical information recording medium side for well correcting sphericalaberration. As X1 defined as described above is plus and its absolutevalue is smaller, or as X2 defined as described above is minus and itsabsolute value is smaller, the effect for overcorrecting the sphericalaberration of the marginal light beam becomes higher, and as X1 definedas described above is plus and its absolute value is larger, or as X2defined as described above is minus and its absolute value is larger,the effect for undercorrecting the spherical aberration of the marginallight beam becomes higher, and accordingly it is necessary that (X1−X2)is within a certain range in order to correct the spherical aberration.From the foregoing, it is preferable to satisfy the formula (16), andthe marginal light beam is not overcorrected excessively when more thanthe lower limit and the marginal light beam is not undercorrectedexcessively when less than the upper limit. In particular, in case of anobjective lens of an infinity type in which the image formationmagnification at the design wavelength λ₀ is zero, it is more preferableto satisfy the following formula:0.40≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.55  (16′)and furthermore, in case of an objective lens of a finite conjugate typewhich converge a diverging light beam emitted from a light source on theinformation recording surface, it is more preferable to satisfy thefollowing formula:0.48≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.63  (16″)

An optical pickup device of claim 21 comprises: a light source; and aconverging optical system including an objective lens for converging alight beam emitted from the light source to an information recordingsurface of an optical information recording medium, and is capable ofrecording and/or reproducing information by converging the light beamemitted from the light source to the information recording surface ofthe optical information recording medium with the converging opticalsystem, wherein the objective lens is a plastic single lens andsatisfies following formulas:NA≧0.8  (1)1.0>f>0.2  (2)

-   -   where NA is an image-side numerical aperture of the objective        lens, which is required for recording and/or reproducing        information to the optical information recording medium and        f (mm) is a focal length of the objective lens.

The operation and effect of this invention is the same as the operationand effect of the invention of claim 1.

An optical pickup device of claim 22 is characterized in that, in theinvention of claim 21, when W(λ₀, T₀) is an RMS value of residualaberration of the objective lens when light having a wavelength of λ₀(nm) which is a design wavelength thereof is incident to the objectivelens at an environmental temperature which is a first ambienttemperature T₀=25° C. and W(λ₀, T₁) is an RMS value of residualaberration of the objective lens when light having the wavelength of λ₀(nm) which is a design wavelength thereof is incident to the objectivelens at the environmental temperature which is a second ambienttemperature T₁=55° C., ΔW defined byΔW=|W(λ₀ , T ₁)−W(λ₀ , T ₀)|  (3)satisfies a following formula:ΔW<0.035 λrms  (4)

The operation and effect of this invention is the same as the operationand effect of the invention of claim 2.

An optical pickup device of claim 23 is characterized in that, in theinvention of claim 21 or 22, the design wavelength λ₀ of the opticalobjective lens is not more than 500 nm, and in case that fB(λ₀, T₀) is aback focal length of the objective lens when light having a wavelengthof λ₀ (nm) is incident to the objective lens at an environmentaltemperature which is a first ambient temperature T₀=25° C. and fB(λ₁,T₀) is a back focal length of the objective lens when light having awavelength of λ₁ (nm) which is 5 nm longer than the wavelength of λ₀ isincident to the objective lens at the environmental temperature which isthe first ambient temperature T₀=25° C., ΔfB defined byΔfB=|fB(λ₁ , T ₀)−fB(λ₀ , T ₀)|  (5)satisfies a following formula:ΔfB<0.001 mm  (6)

The operation and effect of this invention is the same as the operationand effect of the invention of claim 3.

An optical pickup device of claim 24 is characterized in that, in theinvention of any one of claims 21 to 23, the objective lens is anobjective lens of a finite conjugate type for converging a diverginglight beam emitted from the light source to the information recordingsurface of the optical information recording medium and satisfies afollowing formula:0.8>f>0.2  (6A)

The operation and effect of this invention is the same as the operationand effect of the invention of claim 4.

An optical pickup device of claim 25 is characterized in that, in theinvention of claim 24, m satisfies a following formula when m is animage formation magnification of the objective lens:0.2>|m|>0.02  (6B)

The operation and effect of this invention is the same as the operationand effect of the invention of claim 5.

An optical pickup device of claim 26 is characterized in that, in theinvention of claim 24 or 25, the objective lens and the light source areunited by an actuator at least to be driven for tracking.

In an objective lens of an finite conjugate type to which a diverginglight beam is made incident, coma aberration generated by tracking erroris a problem. This reason is that the emitting point becomes an off-axisobject point for the objective lens when the objective lens isdecentered from the emitting point of light source by tracking error.Though decentering of the objective lens due to tracking error is about0.2 to 0.3 mm in an ordinary optical pickup device, the objective lensof the present invention is a lens having a short focal lengthsatisfying the above-described formula (6A), and accordingly comaaberration and astigmatic aberration are generated significantly and itis impossible to perform good recording/reproducing to an opticalinformation recording medium when the objective lens is decentered byabout 0.2 to 0.3 mm by tracking error. Therefore, the optical pickupdevice of claim 22 was configured such that the objective lens and thelight source are united by an actuator at least to be driven fortracking. Thereby, it is possible to solve the problem that comaaberration and astigmatic aberration are generated by tracking error.

An optical pickup device of claim 27 comprises: a light source; and aconverging optical system including an objective lens for converging alight beam emitted from the light source to an information recordingsurface of an optical information recording medium, and is capable ofrecording and/or reproducing information by converging the light beamemitted from the light source to the information recording surface ofthe optical information recording medium with the converging opticalsystem,

-   -   wherein the objective lens is a plastic single lens that        comprises a ring-shaped phase structure on at least one optical        surface, the ring-shaped phase structure comprising a plurality        of ring surfaces and formed so that adjacent ring surfaces        generate a predetermined optical path difference for incident        light, and satisfies following formulas:        NA≧0.8  (7)        1.3>f>0.2  (8)        where NA is an image-side numerical aperture of the objective        lens, which is required for recording and/or reproducing        information for the optical information recording medium and        f (mm) is a focal length of the objective lens.

The operation and effect of this invention is the same as the operationand effect of claim 6.

An optical pickup device of claim 28 is characterized in that, in theinvention of claim 27, the ring-shaped phase structure is a diffractionstructure having a function for diffracting predetermined incident lightand the objective lens forms a converging wave front which is convergedon the information recording surface owing to an effect obtained bycombining a diffraction effect and a refraction effect. The operationand effect of this invention is the same as the operation and effect ofclaim 7.

An optical pickup device of claim 29 is characterized in that, in theinvention of claim 28, the objective lens has spherical aberrationcharacteristics that spherical aberration changes in an undercorrecteddirection when a wavelength of the incident light changes to a longerwavelength. The operation and effect of this invention is the same asthe operation and effect of claim 8.

An optical pickup device of claim 30 is characterized in that, in theinvention of claim 28 or 29, when an optical path difference added to awave front transmitted through the diffraction structure is denoted byan optical path difference function Φ_(b) defined byΦ_(b) =b ₂ ·h ² +b ₄ ·h ⁴ +b ₆ ·h ⁶+ . . .(wherein b₂, b₄, b₆ . . . are 2nd-order, 4th-order, 6th-order . . .optical path difference function coefficients, respectively), afollowing formula is satisfied:−70<(b ₄ ·h _(max) ⁴)/(f·λ ₀·10⁻⁶·(NA·(1−m))⁴)<−20  (8A)wherein λ₀ (nm) is a design wavelength of the objective lens, h_(MAX) isan effective diameter maximum height (mm) of the optical surface onwhich the diffraction structure is formed and m is an image formationmagnification of the objective lens. The operation and effect of thisinvention is the same as the operation and effect of claim 9.

An optical pickup device of claim 31 is characterized in that, in theinvention of claim 27, the ring-shaped phase structure generates thepredetermined optical path difference for the incident light by formingthe adjacent ring surfaces so as to be displaced in an optical axisdirection each other, and the objective lens forms a converging wavefront which is converged on the information recording surface owing to arefraction effect. The operation and effect of this invention is thesame as the operation and effect of claim 10.

The optical pickup device of claim 32 is characterized in that, in theinvention of claim 31, when a ring surface including an optical axis iscalled a central ring surface, a ring surface adjacent to an outside ofthe central ring surface is formed to be displaced in the optical axisdirection so as to have a shorter optical path length than the centralring surface, a ring surface at a maximum effective diameter position isformed to be displaced in the optical axis direction so as to have alonger optical path length than an ring surface adjacent to an insidethereof, and a ring surface at a position of 75% of a maximum effectivediameter is formed to be displaced so as to have a shorter optical pathlength than a ring surface adjacent to an inside thereof and a ringsurface adjacent to an outside thereof. The operation and effect of thisinvention is the same as the operation and effect of claim 11.

An optical pickup device of claim 33 is characterized in that, in theinvention of claim 21 or 22, a total of the ring surfaces is from 3 to20. The operation and effect of this invention is the same as theoperation and effect of claim 12.

An optical pickup device of claim 34 is characterized in that, in theinvention of any one of claims 21 to 23, when Δ_(j) (μm) is a stepamount of an arbitrary step of steps in the optical axis direction at aboundary of mutually adjacent ring surfaces in a ring-shaped phasestructure formed in a region from a height of 75% to a height of 100% ofan effective diameter maximum height of the optical surface on which thering-shaped phase structure is formed and n is a refractive index of theobjective lens at a design wavelength of λ₀ (nm), m_(j) represented bym _(j) =INT(X)  (8B)(wherein X=Δ_(j)·(n−1)/(λ₀·10⁻³) and INT(X) is an integer obtained byhalf adjust of X) is an integer not less than 2. The operation andeffect of this invention is the same as the operation and effect ofclaim 13

An optical pickup device of 35 is characterized in that, in theinvention of any one of claims 27 to 34, when W(λ₀, T₀) is an RMS valueof residual aberration of the objective lens when light having awavelength of λ₀ (nm) which is a design wavelength thereof is incidentto the objective lens at an environmental temperature which is a firstambient temperature T₀=25° C., W(λ₁, T₀) is an RMS value of residualaberration of the objective lens when light having a wavelength of λ₁(nm) which is 5 nm longer than the wavelength of λ₀ is incident to theobjective lens at the environmental temperature which is the firstambient temperature T₀=25° C. and W(λ₂, T₁) is an RMS value of residualaberration of the objective lens when light having a wavelength of λ₂(nm) is incident to the objective lens at the environmental temperaturewhich is a second ambient temperature T₁=55° C., ΔW1 and ΔW2 defined byΔW1=|W(λ₂ , T ₁)−W(λ₀ , T ₀)|  (9)ΔW2=|W(λ₁ , T ₀)−W(λ₀ , T ₀)|  (10)satisfy following formulas:ΔW1<0.035 λrms  (11)ΔW2<0.035 λrms  (12)wherein

-   when λ₀<600 nm, λ₂=λ₀+1.5 (nm) and-   when λ₀≧600 nm, λ₂=λ₀+6 (nm).

The operation and effect of this invention is the same as the operationand effect of claim 14.

Preferably, an optical pickup device of 36 satisfies a following formulain the invention of claim 35:{square root}((ΔW1)²+(ΔW2)²)<0.05 λrms  (13)

The operation and effect of this invention is the same as the operationand effect of claim 15.

In an optical pickup device of claim 37, in the present invention of anyone of claims 27 to 36, the objective lens is an objective lens of afinite conjugate type for converging a diverging light beam emitted fromthe light source on the information recording surface and satisfies afollowing formula:1.1>f>0.2  (13A)The operation and effect of this invention is the same as the operationand effect of claim 16.

An optical pickup device of claim 38 satisfies a following formula whenm is an image formation magnification of the objective lens in theinvention of claim 37:0.2>|m|>0.02  (13B)The operation and effect of this invention is the same as the operationand effect of claim 17.

An optical pickup device of claim 39 is characterized in that, in theinvention of claim 37 or 38, the objective lens and the light source areunited by an actuator at least to be driven for tracking. The operationand effect of this invention is the same as the operation and effect ofclaim 26.

An optical pickup device of claim 40, satisfies a following formula inthe invention of any one of claims 21 to 39:0.8<d/f<1.8  (14)

-   -   where d (mm) is a lens thickness in an optical axis of the        objective lens and f (mm) is a focal length.

The operation and effect of this invention is the same as the operationand effect of claim 18.

An optical pickup device of claim 41 is characterized in that, in theinvention of any one of claims 21 to 40, the design wavelength of λ₀(nm) of the objective lens satisfies a following formula:500≧λ₀≧350  (15)

The operation and effect of this invention is the same as the operationand effect of claim 19.

An optical pickup device of claim 42 satisfies a following formula inthe invention of any one of claims 21 to 41:0.40≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.63  (16)wherein

-   X1: a distance (mm) in an optical axis direction between a plane    that is perpendicular to an optical axis and tangent to a top of an    optical surface on a light source side and an optical surface on the    light source side in a most peripheral portion of an effective    diameter (position of the NA on a surface on the light source side    to which a marginal light beam is made incident), wherein plus is a    case of measurement for a direction of the optical information    recording medium with reference to the tangent plane and minus is a    case of measurement for a direction of the light source,-   X2: a distance (mm) in an optical axis direction between a plane    that is perpendicular to an optical axis and tangent to a top of an    optical surface on an optical information recording medium side and    an optical surface on the optical information recording medium side    in a most peripheral portion of an effective diameter (position of    the NA on a surface on the optical information recording medium side    to which a marginal light beam is made incident), wherein plus is a    case of measurement for a direction of the optical information    recording medium with reference to the tangent plane and minus is a    case of measurement for a direction of the light source,-   N: a refractive index of the objective lens at the design wavelength    of λ₀,-   f: a focal length (mm) of the objective lens, and-   m: an image formation magnification of the objective lens.

The operation and effect of this invention is the same as the operationand effect of claim 20.

An optical information recording/reproducing apparatus of claim 43comprises optical pickup device that comprises: a light source; and aconverging optical-system including an objective lens for converging alight beam emitted from the light source to an information recordingsurface of an optical information recording medium, and is capable ofrecording and/or reproducing information by converging the light beamemitted from the light source to the information recording surface ofthe optical information recording medium with the converging opticalsystem, wherein the objective lens is a plastic single lens andsatisfies following formulas:NA≧0.8  (1)1.0>f>0.2  (2)

-   -   where NA is an image-side numerical aperture of the objective        lens, which is required for recording and/or reproducing        information to the optical information recording medium and        f (mm) is a focal length of the objective lens.

The operation and effect of this invention is the same as the operationand effect of claim 1.

An optical information recording/reproducing apparatus of claim 44 ischaracterized in that, in the invention of claim 43, when W(λ₀, T₀) isan RMS value of residual aberration of the objective lens when lighthaving a wavelength of λ₀ (nm) which is a design wavelength thereof isincident to the objective lens at an environmental temperature which isa first ambient temperature T₀=25° C. and W(λ₀, T₁) is an RMS value ofresidual aberration of the objective lens when light having thewavelength of λ₀ (nm) which is a design wavelength thereof is incidentto the objective lens at the environmental temperature which is a secondambient temperature T₁=55° C., ΔW defined byΔW=|W(λ₀ , T ₁)−W(λ₀ , T ₀)|  (3)satisfies a following formula:ΔW<0.035 λrms  (4)

The operation and effect of this invention is the same as the operationand effect of claim 2.

An optical information recording/reproducing apparatus of claim 45 ischaracterized in that, in the invention of claim 43 or 44, the designwavelength λ₀ of the optical objective lens is not more than 500 nm, andin case that fB(λ₀, T₀) is a back focal length of the objective lenswhen light having a wavelength of λ₀ (nm) is incident to the objectivelens at an environmental temperature which is a first ambienttemperature T₀=25° C. and fB(λ₁, T₀) is a back focal length of theobjective lens when light having a wavelength of λ₁ (nm) which is 5 nmlonger than the wavelength of λ₀ is incident to the objective lens atthe environmental temperature which is the first ambient temperatureT₀=25° C., ΔfB defined byΔfB=|fB(λ₁ , T ₀)−fB(λ₀ , T ₀)|  (5)satisfies a following formula:ΔfB<0.001 mm  (6)

The operation and effect of this invention is the same as the operationand effect of claim 3.

An optical information recording/reproducing apparatus of claim 46 ischaracterized in that, in the invention of any one of claims 43 to 45,the objective lens is an objective lens of a finite conjugate type forconverging a diverging light beam emitted from the light source to theinformation recording surface of the optical information recordingmedium and satisfies a following formula:0.8>f>0.2  (6A)The operation and effect of this invention is the same as the operationand effect of claim 4.

An optical information recording/reproducing apparatus of claim 47 ischaracterized in that, in the invention of claim 46, m satisfies afollowing formula when m is an image formation magnification of theobjective lens:0.2>|m|>0.02  (6B)The operation and effect of this invention is the same as the operationand effect of claim 5.

An optical information recording/reproducing apparatus of claim 48 ischaracterized in that, in the invention of claim 46 or 47, the objectivelens and the light source are united by an actuator at least to bedriven for tracking. The operation and effect of this invention is thesame as the operation and effect of claim 26.

An optical information recording/reproducing apparatus of claim 49comprises an optical pickup device that comprises: a light source; and aconverging optical system including an objective lens for converging alight beam emitted from the light source to an information recordingsurface of an optical information recording medium, and is capable ofrecording and/or reproducing information by converging the light beamemitted from the light source to the information recording surface ofthe optical information recording medium with the converging opticalsystem, wherein the objective lens is a plastic single lens thatcomprises a ring-shaped phase structure on at least one optical surface,the ring-shaped phase structure comprising a plurality of ring surfacesand formed so that adjacent ring surfaces generate a predeterminedoptical path difference for incident light, and satisfies followingformulas:NA≧0.8  (7)1.3>f>0.2  (8)

-   -   where NA is an image-side numerical aperture of the objective        lens, which is required for recording and/or reproducing        information for the optical information recording medium and        f (mm) is a focal length of the objective lens.

The operation and effect of this invention is the same as the operationand effect of claim 6.

An optical information recording/reproducing apparatus of claim 50 ischaracterized in that, in the invention of claim 49, the ring-shapedphase structure is a diffraction structure having a function fordiffracting predetermined incident light and the objective lens forms aconverging wave front which is converged on the information recordingsurface owing to an effect obtained by combining a diffraction effectand a refraction effect. The operation and effect of this invention isthe same as the operation and effect of claim 7.

An optical information recording/reproducing apparatus of claim 51 ischaracterized in that, in the invention of claim 50, the objective lenshas spherical aberration characteristics that spherical aberrationchanges in an undercorrected direction when a wavelength of the incidentlight changes to a longer wavelength. The operation and effect of thisinvention is the same as the operation and effect of claim 8.

An optical information recording/reproducing apparatus of claim 52 ischaracterized in that, in the invention of claim 50 or 51, when anoptical path difference added to a wave front transmitted through thediffraction structure is denoted by an optical path function Φ_(b)defined byΦ_(b) =b ₂ ·h ² +b ₄ ·h ⁴ +b ₆ ·h ⁶+ . . .(wherein b₂, b₄, b₆ . . . are 2nd-order, 4th-order, 6th-order . . .optical path difference function coefficients, respectively), afollowing formula is satisfied:−70<(b ₄ ·h _(max) ⁴)/(f·λ ₀·10⁻⁶·(NA·(1−m))⁴)<−20  (8A)wherein λ₀ (nm) is a design wavelength of the objective lens, h_(MAX) isan effective diameter maximum height (mm) of the optical surface onwhich the diffraction structure is formed and m is an image formationmagnification of the objective lens. The operation and effect of thisinvention is the same as the operation and effect of claim 9.

An optical information recording/reproducing apparatus of claim 53 ischaracterized in that, in the invention of claim 49, the ring-shapedphase structure generates the predetermined optical path difference forthe incident light by forming the adjacent ring surfaces so as to bedisplaced in an optical axis direction each other, and the objectivelens forms a converging wave front which is converged on the informationrecording surface owing to a refraction effect. The operation and effectof this invention is the same as the operation and effect of claim 10.

The optical information recording/reproducing apparatus of claim 54 ischaracterized in that, in the invention of claim 53, when a ring surfaceincluding an optical axis is called a central ring surface, a ringsurface adjacent to an outside of the central ring surface is formed tobe displaced in the optical axis direction so as to have a shorteroptical path length than the central ring surface, a ring surface at amaximum effective diameter position is formed to be displaced in theoptical axis direction so as to have a longer optical path length thanan ring surface adjacent to an inside thereof, and a ring surface at aposition of 75% of a maximum effective diameter is formed to bedisplaced so as to have a shorter optical path length than a ringsurface adjacent to an inside thereof and a ring surface adjacent to anoutside thereof. The operation and effect of this invention is the sameas the operation and effect of claim 11.

An optical information recording/reproducing apparatus of 55 ischaracterized in that, in the invention of claim 53 or 54, a total ofthe ring surfaces is from 3 to 20. The operation and effect of thisinvention is the same as the operation and effect of claim 12.

An optical information recording/reproducing apparatus of claim 56 ischaracterized in that, in the invention of any one of claims 53 to 55,when Δ_(j) (μm) is a step amount of an arbitrary step of steps in theoptical axis direction at a boundary of mutually adjacent ring surfacesin a ring-shaped phase structure formed in a region from a height of 75%to a height of 100% of an effective diameter maximum height of theoptical surface on which the ring-shaped phase structure is formed and nis a refractive index of the objective lens at a design wavelength of λ₀(nm), m_(j) represented bym _(j) =INT(X)  (8B)(wherein X=Δ_(j)·(n−1)/(λ₀·10⁻³) and INT(X) is an integer obtained byhalf adjust of X) is an integer not less than 2. The operation andeffect of this invention is the same as the operation and effect ofclaim 13.

An optical information recording/reproducing apparatus of claim 57 ischaracterized in that, in the invention of any one of claims 49 to 56,in case that W(λ₀, T₀) is an RMS value of residual aberration of theobjective lens when light having a wavelength of λ₀ (nm) which is adesign wavelength thereof is incident to the objective lens at anenvironmental temperature which is a first ambient temperature T₀=25°C., W(λ₁, T₀) is an RMS value of residual aberration of the objectivelens when light having a wavelength of λ₁ (nm) which is 5 nm longer thanthe wavelength of λ₀ is incident to the objective lens at theenvironmental temperature which is the first ambient temperature T₀=25°C., and W(λ₂, T₁) is an RMS value of residual aberration of theobjective lens when light having a wavelength of λ₂ (nm) is incident tothe objective lens at the environmental temperature which is a secondambientΔW1=|W(λ₂ , T ₁)−W(λ₀ , T ₀)|  (9)ΔW2=|W(λ₁ , T ₀)−W(λ₀ , T ₀)|  (10)satisfy following formulas:ΔW1<0.035 λrms  (11)ΔW2<0.035 λrms  (12)wherein

-   when λ₀<600 nm, λ₂=λ₀+1.5 (nm) and-   when λ₀≧600 nm, λ₂=λ₀+6 (nm).

The operation and effect of this invention is the same as the operationand effect of claim 14.

Preferably, an optical information recording/reproducing apparatus ofclaim 58 satisfies a following formula in the invention of claim 47:{square root}((ΔW1)²+(ΔW2)²)<0.05 λrms  (13)

The operation and effect of this invention is the same as the operationand effect of claim 15.

An optical information recording/reproducing apparatus of claim 59 ischaracterized in that, in the invention of any one of claims 49 to 58being an objective lens of a finite conjugate type for converging adiverging light beam emitted from the light source on the informationrecording surface and satisfying a following formula. The operation andeffect of this invention is the same as the operation and effect ofclaim 16.1.1>f>0.2  (13A)

An optical information recording/reproducing apparatus of claim 60satisfies a following formula when m is an image formation magnificationof the objective lens in the invention of claim 59.

The operation and effect of this invention is the same as the operationand effect of claim 17.0.2>|m|>0.02  (13B)

An optical information recording/reproducing apparatus of claim 61 ischaracterized in that, in the invention of claim 59 or 60, the objectivelens and the light source are united by an actuator at least to bedriven for tracking. The operation and effect of this invention is thesame as the operation and effect of claim 26.

An optical information recording/reproducing apparatus of claim 62satisfies a following formula in the present invention of any one ofclaims 43 to 61:0.8<d/f<1.8  (14)

-   -   where d (mm) is a lens thickness in an optical axis of the        objective lens and f (mm) is a focal length.

The operation and effect of this invention is the same as the operationand effect of claim 18.

An optical information recording/reproducing apparatus of claim 63 ischaracterized in that, in the invention of any one of claims 43 to 62,the design wavelength of λ₀ (nm) of the objective lens satisfies afollowing formula:500≧λ₀≧350  (15)

The operation and effect of this invention is the same as the operationand effect of claim 19.

An optical information recording/reproducing apparatus of claim 64satisfies a following formula in the invention of any one of claims 43to 63:0.40≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.63  (16)where

-   X1: a distance (mm) in an optical axis direction between a plane    that is perpendicular to an optical axis and tangent to a top of an    optical surface on a light source side and an optical surface on the    light source side in a most peripheral portion of an effective    diameter (position of the NA on a surface on the light source side    to which a marginal light beam is incident), wherein X1 is plus in a    case of measuring X1 in a direction of the optical information    recording medium with reference to the tangent plane, and minus in a    case of measuring X1 in a direction of the light source,-   X2: a distance (mm) in an optical axis direction between a plane    that is perpendicular to an optical axis and tangent to a top of an    optical surface on an optical information recording medium side and    an optical surface on the optical information recording medium side    in a most peripheral portion of an effective diameter (position of    the NA on a surface on the optical information recording medium side    to which a marginal light beam is incident), wherein X2 is plus in a    case of measuring X2 in a direction of the optical information    recording medium with reference to the tangent plane and minus in a    case of measuring X2 in a direction of the light source,-   N: a refractive index of the objective lens at the design wavelength    of λ₀,-   f: a focal length (mm) of the objective lens, and-   m: an image formation magnification of the objective lens.

The operation and effect of this invention is the same as the operationand effect of claim 20.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an objective lens 1 of the presentembodiment;

FIG. 2 is a schematic view showing an objective lens 4 of the presentembodiment;

FIG. 3 is a view showing a situation of a wave front of a biconvexplastic single lens having two optical surfaces which are aspherical incase of temperature rising from a design temperature by 30° C.;

FIG. 4 is a view schematically showing the configuration of an opticalpickup device (optical information recording/reproducing apparatus) ofthe first embodiment;

FIG. 5 is a view for illustrating a back focal length fB; and

FIG. 6 is a view schematically showing the configuration of an opticalpickup device (optical information recording/reproducing apparatus) ofthe second embodiment.

BEST MODE FOR CARRYING OUT OF THE INVENTION

Hereinafter, embodiments according to the present invention will bedescribed with the use of drawings. FIG. 1 is a schematic view showingan objective lens 1 of the present embodiment, and (A) is a front view,(B) is a side view and (C) is a view expanding the side. The objectivelens 1 is applied to, for example, an optical pickup device forrecording/reproducing a high-density DVD, an MO or the like using ashort wavelength light source such as a blue-violet laser diode, and hasa function for converging laser light emitted from the light source ontoan information recording surface of an optical disk.

The objective lens 1 is a biconvex plastic single lens having twoaspherical optical surfaces 2 and 3. As shown in FIG. 1(A), aring-shaped phase structure as a concentric diffraction structure wherethe optical axis is the center is formed on the optical surface 2. Asshown in FIG. 1(C), the ring-shaped phase structure has a step Δ in thedirection of the optical axis at the boundary of each ring surface as aFresnel lens. Laser light made incident to any ring surface of thering-shaped phase structure is diffracted to the direction that isdetermined by the width of the ring surface in the directionperpendicular to the optical axis (in the present description, such awidth of a ring surface in the direction perpendicular to the opticalaxis referred to as a “ring surface pitch”).

The ring-shaped phase structure has spherical aberration characteristicsthat spherical aberration changes in the undercorrected direction as thewavelength of the incident light increases. Because the refractive indexof plastic single lens decreases owing to temperature rise, thespherical aberration changes in the overcorrected direction. On theother hand, the emission wavelength of a laser diode change in thedirection to become long owing to temperature rise. For example, becausea blue-violet laser diode has change of an emission wavelength by +0.05nm/° C. owing to temperature rise, the wavelength increases by 1.5 nmwhen the temperature rises by +30° C. Accordingly, owing to providing anobjective lens having spherical aberration characteristics thatspherical aberration changes in the undercorrected direction as thewavelength of the incident light increases, change of sphericalaberration that is made overcorrection by change of the refractive indexaccompanying temperature rise can be counterbalanced by change ofspherical aberration that is made undercorrection by change of theemission wavelength of a laser diode due to temperature rise.

In order to correct spherical aberration by diffraction effect of thering-shaped phase structure in this way, it is necessary to generatechromatic spherical aberration designedly. However, when thermalaberration of a plastic single lens having a high NA is tried to becorrected completely, the generation amount of chromatic sphericalaberration must be configured to be large and accordingly, it isimpossible to use a laser diode having the emission wavelength thatdeviates from a standard wavelength owing to manufacturing error. As forthe objective lens 1, in order to decrease the amount of correction ofthermal aberration, the focal length is set to satisfy the formulas (8)or (13A), and additionally, the correction of thermal aberration and thegeneration amount of chromatic spherical aberration are matched so as torespectively satisfy the formulas (11) to (13). Accordingly, theobjective lens 1 is a plastic single lens having a high NA and yet alens having good thermal aberration and chromatic spherical aberration.

FIG. 2 is a schematic view showing an objective lens 4 of anotherembodiment, and (A) is a front view and (B) is a side view. The opticalsurface 2 is, in the same way as the objective lens 1, applied to anoptical pickup device for recording/reproducing a high-density DVD, anMO or the like using a short wavelength light source such as ablue-violet laser diode, and has a function for converging laser lightemitted from the light source onto an information recording surface ofan optical disk.

The objective lens 4 is a biconvex plastic single lens having twoaspherical optical surfaces 5 and 6. As shown in FIG. 2(A), aring-shaped phase structure as a concentric diffraction structure wherethe optical axis is the center is formed on the optical surface 5. Thering-shaped phase structure has a step Δ at the boundary of each ringsurface in the optical axis direction, and each step Δ is determined insuch a way that laser lights transmitted through adjacent ring surfaceshave an optical path difference of wavelength of integral multiple ofthe wavelength in 25° C., which is the design temperature.

Furthermore, as shown in FIG. 2(B), the ring-shaped phase structurecomprises at least one ring surface formed to be displaced in theoptical axis direction so as to have an optical path length shorter thanthe ring surface lying adjacent to the inside thereof, and at least onering surface formed to be displaced in the optical axis direction so asto have an optical path length longer than the ring surface lyingadjacent to the inside thereof, wherein the ring surface formed to bedisplaced in the optical axis direction so as to have an optical pathlength shorter than the ring surface lying adjacent to the insidethereof is formed on the closer side to the optical axis than the ringsurface formed to be displaced in the optical axis direction so as tohave an optical path length longer than the ring surface lying adjacentto the inside thereof. Hereinafter, principle of correction of thermalaberration of the plastic single lens by the ring-shaped phase structuredetermined in this way will be described.

FIG. 3 is a view showing a situation of a wave front of a biconvexplastic single lens having two optical surfaces which are aspherical incase of temperature rising from a design temperature by 30° C., and theabscissa axis denotes an effective radius of the optical surface and theordinate axis denotes an optical path difference. In the plastic singlelens, spherical aberration is generated by the influence of change ofthe refractive index accompanying temperature rise, and a wave front ischanged as line drawing Ag in FIG. 3. Line drawing Bg in FIG. 3 shows anoptical path difference added to a transmitted wave front by thering-shaped phase structure determined as described above, and Linedrawing Cg shows a situation of a wave front transmitted through thering-shaped phase structure and plastic single lens in case oftemperature rising from a design temperature by 30° C. From linedrawings Bg and Cg, it is understood that, owing to counterbalance ofthe wave front transmitted the ring-shaped phase structure and the wavefront of the plastic single lens in case of temperature rising from thedesign temperature by 30° C., a wave front of laser light converged onthe information recording surface of an optical disk becomes a good wavefront with no optical path difference from a broad view and thermalaberration of the plastic single lens is corrected by the ring-shapedphase structure.

In the same way as the case of correcting thermal aberration bydiffraction effect of the ring-shaped phase structure as a diffractionstructure, in the case of correcting thermal aberration of a plasticsingle lens having a high NA by the effect of a ring-shaped phasestructure determined as above, it is impossible to use a laser diodehaving an emission wavelength that deviates from a standard wavelengthby a manufacturing error because trying to completely correct thermalaberration causes the generation amount of chromatic sphericalaberration to be too large.

As for the objective lens 4, in order to decrease the amount ofcorrection of thermal aberration, the focal length is set to satisfy theformulas (8) or (13A), and additionally, the correction of thermalaberration and the generation amount of chromatic spherical aberrationare matched so as to respectively satisfy the formulas (11) to (13).Accordingly, the objective lens 4 is a plastic single lens having a highNA and yet a lens having good thermal aberration and chromatic sphericalaberration in the same way as the objective lens 1.

FIG. 4 is a view schematically showing the configuration of an opticalpickup device (optical information recording/reproducing apparatus)equipped with the objective lens of the present invention. The opticalpickup device 7 comprises a laser diode 8 as a light source and anobjective lens 9.

The laser diode 8 is a GaN-based blue-violet laser diode generatinglight having a wavelength of about 400 nm. A light source generatinglight having a wavelength of about 400 nm may employ an SHG blue-violetlaser diode, besides the above-described GaN-based blue-violet laserdiode.

The objective lens 9 is any one of a plastic single lens whose focallength satisfies the formula (2), the objective lens 1 of FIG. 1 and theobjective lens 4 of FIG. 2. The objective lens 9 comprises a flangeportion 9A extending perpendicularly to the optical axis. The objectivelens 9 can be attached to the optical pickup device 7 accurately by theflange portion 9A. The numerical aperture of the objective lens 9 on theside of an optical disk 10 is made not less than 0.80.

A diverging light beam emitted from the laser diode 8 is transmittedthrough a polarization beam splitter 11 and passes through a collimatinglens 12 and quarter-wave plate 13 to become a circularly polarizedparallel light beam, and subsequently, has the light beam diameterregulated by a stop 14 and is made a spot that is passed through aprotective layer 10A of the optical disk 10 of a high-density DVD andformed on an information recording surface 10B by the objective lens 9.As for the objective lens 9, focus control and tracking control areperformed by an actuator 15 disposed around it.

A reflected beam light modulated by information bit in the informationrecording surface 10B is transmitted again through the objective lens 9,the stop 14, the quarter-wave plate 13 and the collimating lens 12, andsubsequently, is made a converged light beam, reflected by thepolarization beam splitter 11, provided with astigmatic aberration bypassing through a cylindrical lens 16 and concave lens 17, and convergedon an optical detector 18. Subsequently, it is possible to readinformation recorded on the optical disk 10 by using an output signal ofthe optical detector 18.

FIG. 6 is a view schematically showing the configuration of anotheroptical pickup device (optical information recording/reproducingapparatus) equipped with an objective lens of the present invention. Theoptical pickup device 7′ comprises a laser diode 8 as a light source andan objective lens 9.

The laser diode 8 is a GaN-based blue-violet laser diode generatinglight having a wavelength of about 400 nm. A light source generatinglight having a wavelength of about 400 nm may employ an SHG blue-violetlaser diode, besides the above-described GaN-based blue-violet laserdiode.

The objective lens 9 is any one of a plastic single lens whose focallength satisfies the formula (6A), the above-described objective lens 1of FIG. 1 and the objective lens 4 of FIG. 2. The objective lens 9 is anobjective lens of the finite conjugate type for converging a diverginglight beam emitted from the laser diode 8 on an information recordingsurface 10B through a protective layer 10A of the optical disk 10 of ahigh-density DVD. The objective lens 9 comprises a flange portion 9Aextending perpendicularly to the optical axis. The objective lens 9 canbe attached to the optical pickup device 7′ accurately by the flangeportion 9A. The numerical aperture of the objective lens 9 on the sideof an optical disk 10 is made not less than 0.80.

A diverging light beam emitted from the laser diode 8 is transmittedthrough a polarization beam splitter 11 and passes through aquarter-wave plate 13 to become circularly polarized light, andsubsequently, has the light beam diameter regulated by a stop 14 and ismade a spot that is passed through the protective layer 10A of theoptical disk 10 of a high-density DVD and formed on the informationrecording surface 10B by the objective lens 9. A reflected beam lightmodulated by information bit in the information recording surface 10B istransmitted again through the objective lens 9, the stop 14 and thequarter-wave plate 13, and subsequently, is reflected by thepolarization beam splitter 11, provided with astigmatic aberration bypassing through a cylindrical lens 16 and concave lens 17, and convergedon an optical detector 18. Subsequently, it is possible to readinformation recorded on the optical disk 10 by using an output signal ofthe optical detector 18.

In the optical pickup device 7′, the laser diode 8, objective lens 9,polarization beam splitter 11, quarter-wave plate 13, cylindrical lens16, concave lens 17 and optical detector 18 are modularized onto asubstrate. In tracking control, these are monolithically driven by theactuator 19.

Next, six Examples preferred for the above-described embodiments will beproposed. Examples 1 to 6 are objective lenses applied to an opticalpickup device for a high-density DVD in which the wavelength used forrecording/reproducing information is 405 nm and the thickness of theprotective layer is 0.1 mm. Example 1 is a plastic single lens where thegeneration amounts of thermal aberration and longitudinal chromaticaberration are suppressed low by setting the focal length so as tosatisfy the formula (2), and both of Examples 2 and 3 are plastic singlelenses where the thermal aberration is corrected by the effect of thering-shaped phase structure formed on the first surface (the opticalsurface on the side of the light source). Example 4 is a plastic singlelens of the finite conjugate type where the generation amounts ofthermal aberration and longitudinal chromatic aberration are suppressedlow by setting the focal length so as to satisfy the formula (6A), andboth of Examples 5 and 6 are plastic single lenses of the finiteconjugate type where the thermal aberration is corrected by the effectof the ring-shaped phase structure formed on the first surface (theoptical surface on the side of the light source).

Table 4 shows lens data of the objective lens of Example 1, Table 5shows lens data of the objective lens of Example 2 and Table 6 showslens data of the objective lens of Example 3. In the lens data of Tables4, 5 and 6, r (mm) denotes a curvature radius, d (mm) denotes a surfacedistance, N405 denotes a refractive index at a wavelength of 405 nm andνd denotes an Abbe number at the d-line. TABLE 4 Surface No. r(mm) d(mm)N405 νd Remarks 0 ∞ Light source 1   0.3353 0.6600 1.56013 56.7Objective 2 −0.3615 0.0762 lens 3 ∞ 0.1000 1.61950 30.0 Protective 4 ∞layer Aspherical surface coefficients 1st surface 2nd surface κ−6.9542E−01   −1.7907E+01 A4 7.9891E−01   1.5728E+01 A6 1.3935E+00−3.5161E+02 A8 3.3472E+01   3.4150E+03 A10 −7.8778E+01   −1.3187E+04 A12−7.8324E+02   −5.2600E+02 A14 6.6992E+03 A16 3.4753E+04 A18−1.9498E+05   A20 −5.8872E+05  

TABLE 5 Surface No. r(mm) d(mm) N405 νd Remarks 0 ∞ Light source 1  0.6157 1.1400 1.56013 56.7 Objective 2 −0.9615 0.2018 lens 3 ∞ 0.10001.61950 30.0 Protective 4 ∞ layer Aspherical surface coefficients 1stsurface 2nd surface κ −6.3213E−01 −4.7996E+01 A4   5.0716E−02  1.7646E+00 A6   5.2621E−02 −9.5272E+00 A8   5.2319E−01   1.8626E+01A10 −7.1277E−01   1.2599E+00 A12 −9.9374E−01 −4.0506E+01 A14  3.4591E+00 A16 −2.1262E+00 A18   3.3120E+00 A20 −8.7979E+00Diffraction surface coefficients 1st surface b2 −2.4634E−02 b4−5.1397E−02 b6   5.7231E−02 b8 −9.6553E−02 b10 −4.4043E−02

TABLE 6 Surface No. r(mm) d(mm) N405 νd Remarks 0 ∞ Light source 1 Seebelow See below 1.56013 56.7 Objective 2 −0.5177 0.1190 lens 3 ∞ 0.10001.61950 30.0 Protective 4 ∞ layer Aspherical surface coefficients 1stsurface Ring surface No. 1 2 3 4 5 6 2nd surface Starting 0.000 0.1800.250 0.320 0.468 0.490 height (mm) End 0.180 0.250 0.320 0.468 0.4900.500 height (mm) r(mm) 0.4098 0.4091 0.4088 0.4085 0.4091 0.4093 d(mm)0.790000 0.791446 0.792892 0.794338 0.792892 0.791446 κ −6.8225E−01−6.8654E−01   −6.8173E−01   −6.4004E−01 −6.8027E−01 −6.8136E−01−2.7583E+01 A4   4.7569E−01 4.6142E−01 4.7673E−01   4.3957E−01  4.8637E−01   4.8269E−01   5.6751E+00 A6 −7.5261E−02 3.0402E−014.1806E−02 −7.5127E−01 −5.4146E−02 −6.5495E−02 −6.8184E+01 A8  9.7904E+00 8.5608E+00 8.5753E+00   1.2831E+01   9.7795E+00  9.7619E+00   3.5710E+02 A10 −2.9021E+00 −8.6205E+00   1.3381E+00−1.0574E+01 −3.3573E+00 −3.2629E+00 −7.5179E+02 A12 −1.3130E+02−1.3130E+02   −1.3130E+02   −1.3130E+02 −1.3131E+02 −1.3130E+02−4.5183E+01 A14   2.4202E+02 2.4202E+02 2.4202E+02   2.4202E+02  2.4202E+02   2.4201E+02 A16   2.2763E+03 2.2763E+03 2.2763E+03  2.2763E+03   2.2763E+03   2.2763E+03 A18 −2.2834E+03 −2.2834E+03  −2.2834E+03   −2.2834E+03 −2.2834E+03 −2.2834E+03 A20 −1.8263E+04−1.8263E+04   −1.8263E+04   −1.8263E+04 −1.8263E+04 −1.8263E+04

Example 1 is a plastic single lens having an incident light beamdiameter of 0.8 mm, a focal length f=0.47 mm, an NA of 0.85, a designwavelength of 405 nm, a design temperature of 25° C. Because the focallength is set so as to satisfy the formula (2), it is a plastic singlelens having a high NA and yet a lens where both spherical aberrations ingeneration of thermal aberration and mode hopping are good as shown inTable 7. TABLE 7 thermal Mode aberration(+30° C.) hopping(+1 nm) Example1 0.020 λrms 0.028 λrms

In Table 7, for calculating the thermal aberration, the change rate ofthe refractive index accompanying temperature rise of the plastic lensis −9.0×10⁻⁵ and the change rate of the wavelength of incident lightaccompanying temperature rise is +0.05 nm/° C. For calculating thespherical aberration in generation of mode hopping, the variation of thewavelength of a blue-violet laser diode owing to the mode hopping isassumed +1 nm and the focal position of the objective lens is fixed atthe best image surface position of 405 nm.

As for the objective lens of Example 1, a value of ΔW (formula (3)) isΔW=0.019 λrms because of W(λ₀, T₀)=0.001 λrms (λ₀=405 nm and T₀=25° C.)and W(λ₀, T₁)=0.020 λrms (λ₀=405 nm and T₁=55° C.). A value of ΔfB(formula (5)) is ΔfB=0.0004 mm because of fB(λ₀, T₀)=0.0762 mm (λ₀=405nm and T₀=25° C.) and fB(λ₁, T₀)=0.0766 mm (λ₁=410 nm and T₀=25° C.). Asshown in FIG. 5, a back focal length fB in the present descriptionindicates the distance along the optical axis between the opticalsurface S2 of the objective lens on the side of the optical informationrecording medium and a light beam incident surface S_(IN) of the opticalinformation recording medium.

Example 2 is a plastic single lens having an incident light beamdiameter of 1.5 mm, focal length f=0.88 mm, an NA of 0.85, a designwavelength of 405 nm, a design temperature of 25° C., and a suitableobjective lens as the objective lens 1 in the above-describedembodiment. As shown in Table 8, 80 lengths of ring-shaped phasestructure as a diffraction structure whose boundaries comprise a step Δof about 0.7 μm to 1.2 μm in the optical axis direction are formedwithin the effective diameter on the 1st surface of the objective lensof Example 2. When laser light from the blue-violet laser diode is madeincident to the ring-shaped phase structure, the 1st order diffractedlight is generated so as to have the maximum diffracted light quantity(i.e. the ring-shaped phase structure is optimized at a wavelength of405 nm and a diffraction order of 1). By the diffraction effect of thering-shaped phase structure, thermal aberration is well corrected. TABLE8 Ring Starting End height surface No. height (mm) (mm) 1 0.000 0.126 20.126 0.176 3 0.176 0.213 4 0.213 0.243 5 0.243 0.269 6 0.269 0.291 70.291 0.312 8 0.312 0.330 9 0.330 0.347 10 0.347 0.363 11 0.363 0.378 120.378 0.392 13 0.392 0.406 14 0.406 0.418 15 0.418 0.430 16 0.430 0.44117 0.441 0.452 18 0.452 0.463 19 0.463 0.473 20 0.473 0.482 21 0.4820.491 22 0.491 0.500 23 0.500 0.509 24 0.509 0.517 25 0.517 0.525 260.525 0.533 27 0.533 0.540 28 0.540 0.547 29 0.547 0.554 30 0.554 0.56131 0.561 0.568 32 0.568 0.574 33 0.574 0.580 34 0.580 0.586 35 0.5860.592 36 0.592 0.598 37 0.598 0.603 38 0.603 0.609 39 0.609 0.614 400.614 0.619 41 0.619 0.624 42 0.624 0.629 43 0.629 0.633 44 0.633 0.63845 0.638 0.642 46 0.642 0.647 47 0.647 0.651 48 0.651 0.655 49 0.6550.659 50 0.659 0.663 51 0.663 0.667 52 0.667 0.671 53 0.671 0.675 540.675 0.678 55 0.678 0.682 56 0.682 0.685 57 0.685 0.689 58 0.689 0.69259 0.692 0.695 60 0.695 0.699 61 0.699 0.702 62 0.702 0.705 63 0.7050.708 64 0.708 0.711 65 0.711 0.714 66 0.714 0.717 67 0.717 0.719 680.719 0.722 69 0.722 0.725 70 0.725 0.727 71 0.727 0.730 72 0.730 0.73373 0.733 0.735 74 0.735 0.738 75 0.738 0.740 76 0.740 0.743 77 0.7430.745 78 0.745 0.747 79 0.747 0.750 80 0.750 0.752

As for the objective lens of Example 2, a value of ΔW1 (formula (9)) isΔW1=0.019 λrms because of W(λ₀ T₀)=0.001 λrms (λ₀=405 nm and T₀=25° C.)and W(λ₂, T₁)=0.020 λrms (λ₂=406.5 nm and T₁=55° C.). A value of ΔW2(formula (10)) is ΔW2=0.021 λrms because of W(λ₀, T₀)=0.001 λrms (λ₀=405nm and T₀=25° C.) and W(λ₁, T₀)=0.022 λrms (λ₂=410 nm and T₀=25° C.). Avalue of the formula (8A) in Example 2 is −42.

Example 3 is a plastic single lens having an incident light beamdiameter of 1.0 mm, a focal length f=0.59 mm, an NA of 0.85, a designwavelength of 405 nm, a design temperature of 25° C., and a suitableobjective lens as the objective lens 4 in the above-describedembodiment. As shown in Table 6, 6 lengths of ring-shaped phasestructure as a diffraction structure whose boundaries comprise a step Δof about 1.5 μm to 2.3 μm in the optical axis direction are formedwithin the effective diameter on the 1st surface of the objective lensof Example 3. When laser light from the blue-violet laser diode is madeincident to the ring-shaped phase structure, the 1st order diffractedlight is generated so as to have the maximum diffracted light quantity(i.e. the ring-shaped phase structure is optimized at a wavelength of405 nm and a diffraction order of 1). By the effect of the ring-shapedphase structure, thermal aberration is well corrected.

As for the objective lens of Example 3, a value of ΔW1 (formula (9)) isΔW1=0.013 λrms because of W (λ₀, T₀)=0.002 λrms (λ₀=405 nm and T₀=25°C.) and W(λ₂, T₁)=0.015 λrms (2=406.5 nm and T₁=55° C.). A value of ΔW2(formula (10)) is ΔW2=0.013 λrms because of W (λ₀, T₀)=0.002 λrms(λ₀=405 nm and T₀=25° C.) and W(λ₁, T₁)=0.015 λrms (λ₂=410 nm and T₀=25°C.). As for values of the formula (8B) in Example 3, the 5th ringsurface is m_(j)=3 and the 6th ring surface is m_(j)=3.

Both objective lenses of Examples 2 and 3 have a focal length set so asto satisfy the formula (8) in order to reduce the correction amount ofthermal aberration, and additionally, have a configuration where thecorrection of thermal aberration and the generation amount of chromaticspherical aberration are matched so as to respectively satisfy theformulas (11) to (13). Accordingly, they are plastic single lenses witha high NA and yet a lens having good thermal aberration and chromaticspherical aberration as shown in Table 9. TABLE 9 thermal aberrationchromatic spherical (+30° C.) aberration (+5 nm) Example 2 0.020 λrms0.022 λrms Example 3 0.015 λrms 0.015 λrms

In Table 9, for calculating the thermal aberration, the change rate ofthe refractive index accompanying temperature rise of the plastic lensis −9.0×10⁻⁵ and the change rate of the wavelength of incident lightaccompanying temperature rise is +0.05 nm/° C.

Table 11 shows lens data of the objective lens of Example 5 and Table 15shows lens data of the objective lens of Example 6. In the lens data ofTables 10, 11 and 15, r (mm) denotes a curvature radius, d (mm) denotesa surface distance, N405 denotes a refractive index at a wavelength of405 nm and νd denotes an Abbe number at the d-line. TABLE 10 [Example 4]Surface No. r(mm) d(mm) N405 νd Remarks 0 3.7500 Light source 1   0.21360.3750 1.56013 56.7 Objective 2 −0.2910 0.0742 lens 3 ∞ 0.1000 1.6195030.0 Protective 4 ∞ layer Aspherical surface coefficients 1st surface2nd surface κ −6.5380E−01   −2.9101E−01 A4 5.9438E−01 −8.1619E+00 A62.0735E+01   3.9794E+01 A8 −2.1582E+01   −1.4824E+03 A10 2.8863E+03  3.2709E+04 A12 5.9020E+03 −4.4513E+05 A14 −5.2839E+05     3.4193E+06A16 −1.7610E+06   −1.1349E+07 A18 4.3204E+07 A20 −3.1642E+08  

TABLE 11 [Example 5] Surface No. r(mm) d(mm) N405 νd Remarks 0 5.0000Light source 1   0.2769 0.5240 1.56013 56.7 Objective 2 −0.3763 0.1000lens 3 ∞ 0.1000 1.61950 30.0 Protective 4 ∞ layer Aspherical surfacecoefficients 1st surface 2nd surface κ −6.8145E−01 −9.4697E+00 A4−3.0262E−02   1.9844E+01 A6   1.1148E+00 −4.5104E+02 A8 −1.5150E+01  6.0959E+03 A10   3.1738E+02 −5.2115E+04 A12   2.4517E+02   2.5579E+05A14 −1.9895E+04 −5.5335E+05 A16 −2.8487E+03 A18   9.0622E+05 A20−4.9449E+06 Diffraction surface coefficients 1st surface b4 −2.6238E−01b6 −1.9998E+00

TABLE 15 thermal aberration chromatic spherical (+30° C.) aberration (+5nm) Example 5 0.018 λrms 0.019 λrms

Example 4 is a plastic single lens having a focal length of 0.30 mm, anNA of 0.85, a design wavelength of 405 nm, an image formationmagnification of −0.084 and a design temperature of 25° C. In case ofdisposing a stop regulating a light beam at the surface top position ofthe 1st surface in the objective lens of Example 4, its stop diameterbecomes 0.532 mm. Because the focal length is set so as to satisfy theformula (6A), it is a plastic single lens of finite conjugate type witha high NA and yet a lens where both spherical aberrations in generationof thermal aberration and mode hopping are good as shown in Table 12.TABLE 12 Surface No. r(mm) d(mm) N405 νd Remarks 0 5.000 Light source 1See below See below 1.56013 56.7 Objective 2 −0.3424 0.0956 lens 3 ∞0.1000 1.61950 30.0 Protective 4 ∞ layer Aspherical surface coefficients1st surface Ring surface NO. 1 2 3 4 5 6 7 2nd surface Starting 0.0000.100 0.145 0.180 0.215 0.315 0.338 height (mm) End 0.100 0.145 0.1800.215 0.315 0.338 0.373 height(mm) r(mm) 0.2812 0.2808 0.2806 0.28040.2798 0.2817 0.2796 d(mm) 0.540000 0.541446 0.542912 0.544382 0.5458180.543517 0.540000 κ −6.5614E−01 −6.7753E−01 −6.9605E−01 −6.7324E−01−6.6181E−01 −6.5835E−01 −6.7741E−01 −9.1512E+00 A4   2.2680E−01  3.4429E−01   5.1719E−01   3.7786E−01   2.8692E−01   2.6051E−01  6.6914E−02   1.9167E+01 A6   4.4574E+00   4.4574E+00   4.4574E+00  4.4574E+00   4.4574E+00   4.6743E+00   6.1404E+00 −4.5257E+02 A8−1.4000E+01 −1.4000E+01 −1.4000E+01 −1.4000E+01 −1.4000E+01 −1.4000E+01−1.4000E+01   6.1555E+03 A10   3.2216E+02   3.2216E+02   3.2216E+02  3.2216E+02   3.2216E+02   3.2216E+02   3.2216E+02 −5.1764E+04 A12  5.1481E+02   5.1481E+02   5.1481E+02   5.1481E+02   5.1481E+02  5.1481E+02   5.1481E+02   2.4522E+05 A14 −1.9311E+04 −1.9311E+04−1.9311E+04 −1.9311E+04 −1.9311E+04 −1.9311E+04 −1.9311E+04 −5.0382E+05A16 −1.2688E+04 −1.2688E+04 −1.2688E+04 −1.2688E+04 −1.2688E+04−1.2688E+04 −1.2688E+04 A18   8.2257E+05   8.2257E+05   8.2257E+05  8.2257E+05   8.2257E+05   8.2257E+05   8.2257E+05 A20 −5.0807E+06−5.0807E+06 −5.0807E+06 −5.0807E+06 −5.0807E+06 −5.0807E+06 −5.0807E+06

In Table 12, for calculating the thermal aberration, the change rate ofthe refractive index accompanying temperature rise of the plastic lensis −9.0×10⁻⁵ and the change rate of the wavelength of incident lightaccompanying temperature rise is +0.05 nm/° C. For calculating thespherical aberration in generation of mode hopping, the variation of thewavelength of a blue-violet laser diode owing to the mode hopping isassumed +1 nm and the focal position of the objective lens is fixed atthe best image surface position of 405 nm.

As for the objective lens of Example 4, a value of ΔW (formula (3)) isΔW=0.028 λrms because of W(λ₀, T₀)=0.000 λrms (λ₀=405 nm and T₀=25° C.)and W(λ₀, T₁)=0.028 λrms (λ₀=405 nm and T₁=55° C.). A value of ΔfB(formula (5)) is ΔfB=0.0004 mm because of fB(λ₀, T₀)=0.0742 mm (λ₀=405nm and T₀=25° C.) and fB(λ₁, T₀)=0.0746 mm (λ₁=410 nm and T₀=25° C.).

Example 5 is a plastic single lens having a focal length f=0.40 mm, anNA of 0.85, a design wavelength of 405 nm, an image formationmagnification of −0.083 and a design temperature of 25° C. and asuitable objective lens as the objective lens 1 in the above-describedembodiment. In case of disposing a stop regulating a light beam at thesurface top position of the 1st surface in the objective lens of Example5, its stop diameter becomes 0.708 mm. As shown in Table 13, 27 lengthsof ring-shaped phase structure as a diffraction structure whoseboundaries comprise a step Δ of about 0.7 μm to 1.1 μm in the opticalaxis direction are formed within the effective diameter on the 1stsurface of the objective lens of Example 5. When laser light from theblue-violet laser diode is made incident to the ring-shaped phasestructure, the 1st order diffracted light is generated so as to have themaximum diffracted light quantity (i.e. the ring-shaped phase structureis optimized at a wavelength of 405 nm and a diffraction order of 1 (thediffraction efficiency is the highest). By the diffraction effect of thering-shaped phase structure, thermal aberration is well corrected. TABLE13 thermal aberration Mode (+30° C.) hopping(+1 nm) Example 4 0.028 λrms0.024 λrms

As for the objective lens of Example 5, a value of ΔW1 (formula (9)) isΔW1=0.018 λrms because of W(λ₀, T₀)=0.000 λrms (λ₀=405 nm and T₀=25° C.)and W(λ₂, T₁)=0.018 λrms (λ₂=406.5 nm and T₁=55° C.). A value of ΔW2(formula (10)) is ΔW2=0.019 λrms because of W(λ₀, T₀)=0.000 λrms (λ₀=405nm and T₀=25° C.) and W(λ₁, T₀)=0.019 λrms (λ₁=410 nm and T₀=25° C.).

The objective lens of Example 5 has a focal length set so as to satisfythe formula (13A) in order to reduce the correction amount of thermalaberration, and additionally, have a configuration where the correctionof thermal aberration and the generation amount of chromatic sphericalaberration are matched so as to respectively satisfy the formulas (9) to(11). Accordingly, it is a plastic single lens having a high NA of thefinite conjugate type and yet a lens having good thermal aberration andchromatic spherical aberration as shown in Table 14. TABLE 14 Ringsurface Starting End height No. height (mm) (mm) 1 0.000 0.187 2 0.1870.218 3 0.218 0.238 4 0.238 0.254 5 0.254 0.266 6 0.266 0.277 7 0.2770.286 8 0.286 0.294 9 0.294 0.301 10 0.301 0.308 11 0.308 0.314 12 0.3140.319 13 0.319 0.325 14 0.325 0.330 15 0.330 0.334 16 0.334 0.339 170.339 0.343 18 0.343 0.347 19 0.347 0.351 20 0.351 0.354 21 0.354 0.35822 0.358 0.361 23 0.361 0.364 24 0.364 0.368 25 0.368 0.371 26 0.3710.373 27 0.373 0.376

In Table 14, for calculating the thermal aberration, the change rate ofthe refractive index accompanying temperature rise of the plastic lensis −9.0×10⁻⁵ and the change rate of the wavelength of incident lightaccompanying temperature rise is +0.05 nm/° C. A value of the formula(8A) in Example 5 is −45.

Example 6 is a plastic single lens having a focal length f=0.40 mm, anNA of 0.85, a design wavelength of 405 nm, an image formationmagnification of −0.083 and a design temperature of 25° C. and asuitable objective lens as the objective lens 4 in the above-describedembodiment. In case of disposing a stop regulating a light beam at thesurface top position of the 1st surface in the objective lens of Example6, its stop diameter becomes 0.702 mm. As shown in Table 15, 7 lengthsof ring-shaped phase structure as a diffraction structure whoseboundaries comprise a step Δ of about 1.5 μm to 4.0 μm in the opticalaxis direction are formed within the effective diameter on the 1stsurface of the objective lens of Example 6. By the diffraction effect ofthe ring-shaped phase structure, thermal aberration is well corrected.

As for the objective lens of Example 6, a value of ΔW1 (formula (9)) isΔW1=0.018 λrms because of W(λ₀, T0)=0.002 λrms (λ₀=405 nm and T0=25° C.)and W(λ₂, T₁)=0.020 λrms (λ₂=406.5 nm and T1=55° C.). A value of ΔW2(formula (10)) is ΔW2=0.030 λrms because of W(λ₀, T0)=0.002 λrms (λ₀=405nm and T0=25° C.) and W(λ₁, T₁)=0.032 λrms (λ₁=410 nm and T0=25° C.). Asfor values of the formula (8B) in Example 6, the 6th ring surface ism_(j)=5 and the 7th ring surface is m_(j)=7.

The objective lens of Example 6 has a focal length set so as to satisfythe formula (6A) in order to reduce the correction amount of thermalaberration, and additionally, have a configuration where the correctionof thermal aberration and the generation amount of chromatic sphericalaberration are matched so as to respectively satisfy the formulas (11)to (13). Accordingly, it is a plastic single lens having a high NA ofthe finite conjugate type and yet a lens having good thermal aberrationand chromatic spherical aberration as shown in Table 16. TABLE 16Thermal aberration Chromatic spherical (+30° C.) aberration (+5 nm)Example 6 0.020 λrms 0.032 λrms

In Table 16, for calculating the thermal aberration, the change rate ofthe refractive index accompanying temperature rise of the plastic lensis −9.0×10⁻⁵ and the change rate of the wavelength of incident lightaccompanying temperature rise is +0.05 nm/° C.

A value of the above-described formula (16) in each example, which is{(X1−X2)·(N−1)/(NA·f·{square root}(1+|m |))}, is as follows:

-   -   Example 1: 0.471    -   Example 2: 0.454    -   Example 3: 0.490    -   Example 4: 0.576    -   Example 5: 0.538    -   Example 6: 0.558

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a plasticsingle lens applicable to an objective lens of an optical pickup deviceusing a plastic single lens having a high NA, in which an availabletemperature range is sufficiently broad and converging ability due tomode hopping of a light source is degraded scarcely, and thereby it ispossible to provide an optical pickup device and optical informationrecording/reproducing apparatus having high performance.

1. An objective lens used for an optical pickup device, wherein theoptical pickup device comprises: a light source; and a convergingoptical system including the objective lens for converging a light beamemitted from the light source to an information recording surface of anoptical information recording medium, and the optical pickup device iscapable of recording and/or reproducing information by converging thelight beam emitted from the light source to the information recordingsurface of the optical information recording medium with the convergingoptical system, and wherein the objective lens is a plastic single lensand satisfies following formulas:NA≧0.8  (1)1.0>f>0.2  (2) where NA is an image-side numerical aperture of theobjective lens, which is required for recording and/or reproducinginformation to the optical information recording medium and f (mm) is afocal length of the objective lens.
 2. The objective lens for theoptical pickup device of claim 1, wherein in case that W(λ₀, T₀) is anRMS value of residual aberration of the objective lens when light havinga wavelength of λ₀ (nm) which is a design wavelength thereof is incidentto the objective lens at an environmental temperature which is a firstambient temperature T₀=25° C. and W(λ₀, T₁) is an RMS value of residualaberration of the objective lens when light having the wavelength of λ₀(nm) which is a design wavelength thereof is incident to the objectivelens at the environmental temperature which is a second ambienttemperature T₁=55° C., ΔW defined byΔW=|W(λ₀ , T ₁)−W(λ₀ , T ₀)|  (3) satisfies a following formula:ΔW<0.035 λrms  (4)
 3. The objective lens for the optical pickup deviceof claim 1, wherein the design wavelength λ₀ of the optical objectivelens is not more than 500 nm, and in case that fB(λ₀, T₀) is a backfocal length of the objective lens when light having a wavelength of λ₀(nm) is incident to the objective lens at an environmental temperaturewhich is a first ambient temperature T₀=25° C. and fB(λ₁, T₀) is a backfocal length of the objective lens when light having a wavelength of λ₁(nm) which is 5 nm longer than the wavelength of λ₀ incident to theobjective lens at the environmental temperature which is the firstambient temperature T₀=25° C., ΔfB defined byΔfB=|fB(λ₁ , T ₀)−fB(λ₀ , T ₀)|  (5) satisfies a following formula:ΔfB<0.001 mm  (6)
 4. The objective lens for the optical pickup device ofclaim 1, wherein the objective lens is an objective lens of a finiteconjugate type for converging a diverging light beam emitted from thelight source to the information recording surface of the opticalinformation recording medium and satisfies a following formula:0.8>f>0.2  (6A)
 5. The objective lens for the optical pickup device ofclaim 4, wherein m satisfies a following formula when m is an imageformation magnification of the objective lens:0.2>|m|>0.02  (6B)
 6. An objective lens used for an optical pickupdevice, wherein the optical pickup device comprises a light source; anda converging optical system including an objective lens for converging alight beam emitted from the light source to an information recordingsurface of an optical information recording medium, and the opticalpickup device is capable of recording and/or reproducing information byconverging the light beam emitted from the light source to theinformation recording surface of the optical information recordingmedium with the converging optical system, wherein the objective lens isa plastic single lens that comprises a ring-shaped phase structure on atleast one optical surface, the ring-shaped phase structure comprising aplurality of ring surfaces and formed so that adjacent ring surfacesgenerate a predetermined optical path difference for incident light, andsatisfies following formulas:NA≧0.8  (7)1.3>f>0.2  (8) where NA is an image-side numerical aperture of theobjective lens, which is required for recording and/or reproducinginformation for the optical information recording medium and f (mm) is afocal length of the objective lens.
 7. The objective lens for theoptical pickup device of claim 6, wherein the ring-shaped phasestructure is a diffraction structure having a function for diffractingpredetermined incident light and the objective lens forms a convergingwave front which is converged on the information recording surface owingto an effect obtained by combining a diffraction effect and a refractioneffect.
 8. The objective lens for the optical pickup device of claim 7,wherein the objective lens has a spherical aberration characteristicthat spherical aberration changes in an undercorrected direction when awavelength of the incident light changes to a longer wavelength.
 9. Theobjective lens for the optical pickup device of claim 7, wherein when anoptical path difference added to a wave front transmitted through thediffraction structure is denoted by an optical path difference functionΦ_(b) defined byΦ_(b) =b ₂ ·h ² +b ₄ ·h ⁴ +b ₆ ·h ⁶+ . . . (wherein b₂, b₄, b₆ . . . are2nd-order, 4th-order, 6th-order . . . optical path difference functioncoefficients, respectively), a following formula is satisfied:−70<(b ₄ ·h _(MAX) ⁴)/(f·λ ₀·10⁻⁶·(NA·(1−M))⁴)<−20  (8A) wherein λ₀ (nm)is a design wavelength of the objective lens, h_(MAX) is an effectivediameter maximum height (mm) of the optical surface on which thediffraction structure is formed and m is an image formationmagnification of the objective lens.
 10. The objective lens for theoptical pickup device of claim 6, wherein the ring-shaped phasestructure generates the predetermined optical path difference for theincident light by forming the adjacent ring surfaces so as to bedisplaced in an optical axis direction each other, and the objectivelens forms a converging wave front which is converged on the informationrecording surface owing to a refraction effect.
 11. The objective lensfor the optical pickup device of claim 10, wherein when a ring surfaceincluding an optical axis is called a central ring surface, a ringsurface adjacent to an outside of the central ring surface is formed tobe displaced in the optical axis direction so as to have a shorteroptical path length than the central ring surface, a ring surface at amaximum effective diameter position is formed to be displaced in theoptical axis direction so as to have a longer optical path length thanan ring surface adjacent to an inside thereof, and a ring surface at aposition of 75% of a maximum effective diameter is formed to bedisplaced so as to have a shorter optical path length than a ringsurface adjacent to an inside thereof and a ring surface adjacent to anoutside thereof.
 12. The objective lens for the optical pickup device ofclaim 10, wherein a total of the ring surfaces is from 3 to
 20. 13. Theobjective lens for the optical pickup device of claim 10, wherein whenΔ_(j) (μm) is a step amount of an arbitrary step of steps in the opticalaxis direction at a boundary of mutually adjacent ring surfaces in aring-shaped phase structure formed in a region from a height of 75% to aheight of 100% of an effective diameter maximum height of the opticalsurface on which the ring-shaped phase structure is formed and n is arefractive index of the objective lens at a design wavelength of λ₀(nm), m_(j) represented bym _(j) =INT(X)  (8B) (wherein X=Δ_(j)·(n−1)/(λ₀·10⁻³) and INT(X) is aninteger obtained by half adjust of X) is an integer not less than
 2. 14.The objective lens for the optical pickup device of claim 6, wherein incase that W(λ₀, T₀) is an RMS value of residual aberration of theobjective lens when light having a wavelength of λ₀ (nm) which is adesign wavelength thereof is incident to the objective lens at anenvironmental temperature which is a first ambient temperature T₀=25°C., W(λ₁, T₀) is an RMS value of residual aberration of the objectivelens when light having a wavelength of λ₁ (nm) which is 5 nm longer thanthe wavelength of λ₀ is incident to the objective lens at theenvironmental temperature which is the first ambient temperature T₀=25°C. and W(λ₂, T₁) is an RMS value of residual aberration of the objectivelens when light having a wavelength of λ₂ (nm) is incident to theobjective lens at the environmental temperature which is a secondambient temperature T₁=55° C., ΔW1 and ΔW2 defined byΔW1=|W(λ₂ , T ₁)−W(λ₀ , T ₀)|  (9)ΔW2=|W(λ₁ , T ₀)−W(λ₀ , T ₀)|  (10) satisfy following formulas:ΔW1<0.035 λrms  (11)ΔW2<0.035 λrms  (12) wherein when λ₀<600 nm, λ₂=λ₀+1.5 (nm) and whenλ₀≧600 nm, λ₂=λ₀+6 (nm).
 15. The objective lens for the optical pickupdevice of claim 14, wherein the objective lens satisfies a followingformula:{square root}((ΔW1)²+(ΔW2)²)<0.05 λrms  (13)
 16. The objective lens forthe optical pickup device of claim 6, wherein the objective lens is anobjective lens of a finite conjugate type for converging a diverginglight beam emitted from the light source on the information recordingsurface and satisfies a following formula:1.1>f>0.2  (13A)
 17. The objective lens for the optical pickup device ofclaim 16, satisfying a following formula when m is an image formationmagnification of the objective lens:0.2>|m|>0.02  (13B)
 18. The objective lens for the optical pickup deviceof claim 1, wherein the objective lens satisfies a following formula:0.8<d/f<1.8  (14) where d (mm) is a lens thickness in an optical axis ofthe objective lens and f (mm) is the focal length.
 19. The objectivelens for the optical pickup device of claim 1, wherein the designwavelength of λ₀ (nm) of the objective lens satisfies a followingformula:500≧λ₀≧350  (15)
 20. The objective lens for the optical pickup device ofclaim 1, wherein the objective lens satisfies a following formula:0.40≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.63  (16) where X1: adistance (mm) in an optical axis direction between a plane that isperpendicular to an optical axis and tangent to a top of an opticalsurface on a light source side and an optical surface on the lightsource side in a most peripheral portion of an effective diameter(position of the NA on a surface on the light source side to which amarginal light beam is incident), wherein X1 is plus in a case ofmeasuring X1 in a direction of the optical information recording mediumwith reference to the tangent plane, and minus in a case of measuring X1in a direction of the light source, X2: a distance (mm) in an opticalaxis direction between a plane that is perpendicular to an optical axisand tangent to a top of an optical surface on an optical informationrecording medium side and an optical surface on the optical informationrecording medium side in a most peripheral portion of an effectivediameter (position of the NA on a surface on the optical informationrecording medium side to which a marginal light beam is incident),wherein X2 is plus in a case of measuring X2 in a direction of theoptical information recording medium with reference to the tangent planeand minus in a case of measuring X2 in a direction of the light source,N: a refractive index of the objective lens at the design wavelength ofλ₀, f: the focal length (mm) of the objective lens, and m: an imageformation magnification of the objective lens.
 21. An optical pickupdevice comprising: a light source; and a converging optical systemincluding an objective lens for converging a light beam emitted from thelight source to an information recording surface of an opticalinformation recording medium, and wherein the optical pickup device iscapable of recording and/or reproducing information by converging thelight beam emitted from the light source to the information recordingsurface of the optical information recording medium with the convergingoptical system, wherein the objective lens is a plastic single lens andsatisfies following formulas:NA≧0.8  (1)1.0>f>0.2  (2) where NA is an image-side numerical aperture of theobjective lens, which is required for recording and/or reproducinginformation to the optical information recording medium and f (mm) is afocal length of the objective lens.
 22. The optical pickup device ofclaim 21, wherein in case that W(λ₀, T₀) is an RMS value of residualaberration of the objective lens when light having a wavelength of λ₀(nm) which is a design wavelength thereof is incident to the objectivelens at an environmental temperature which is a first ambienttemperature T₀=25° C., and W(λ₀, T₁) is an RMS value of residualaberration of the objective lens when light having the wavelength of λ₀(nm) which is a design wavelength thereof is incident to the objectivelens at the environmental temperature which is a second ambienttemperature T₁=55° C., ΔW defined byΔW=|W(λ₀ , T ₁)−W(λ₀ , T ₀)|  (3) satisfies a following formula:ΔW<0.035 λrms  (4)
 23. The optical pickup device of claim 21, whereinthe design wavelength λ₀ of the optical objective lens is not more than500 nm, and in case that fB(λ₀, T₀) is a back focal length of theobjective lens when light having a wavelength of λ₀ (nm) is incident tothe objective lens at an environmental temperature which is a firstambient temperature T₀=25° C. and fB(λ₀ T₀) is a back focal length ofthe objective lens when light having a wavelength of λ₁ (nm) which is 5nm longer than the wavelength of λ₀ is incident to the objective lens atthe environmental temperature which is the first ambient temperatureT₀=25° C., ΔfB defined byΔfB=|fB(λ₁ , T ₀)−fB(λ₀ , T ₀)|  (5) satisfies a following formula:ΔfB<0.001 mm  (6)
 24. The optical pickup device of claim 21, wherein theobjective lens is an objective lens of a finite conjugate type forconverging a diverging light beam emitted from the light source to theinformation recording surface of the optical information recordingmedium and satisfies a following formula:0.8>f>0.2  (6A)
 25. The optical pickup device of claim 24, wherein msatisfies a following formula when m is an image formation magnificationof the objective lens:0.2>|m|>0.02  (6B)
 26. The optical pickup device of claim 24, whereinthe objective lens and the light source are united by an actuator atleast to be driven for tracking.
 27. An optical pickup devicecomprising: a light source; and a converging optical system including anobjective lens for converging a light beam emitted from the light sourceto an information recording surface of an optical information recordingmedium, wherein the optical pickup device is capable of recording and/orreproducing information by converging the light beam emitted from thelight source to the information recording surface of the opticalinformation recording medium with the converging optical system, whereinthe objective lens is a plastic single lens that comprises a ring-shapedphase structure on at least one optical surface, the ring-shaped phasestructure comprising a plurality of ring surfaces and formed so thatadjacent ring surfaces generate a predetermined optical path differencefor incident light, and satisfies following formulas:NA≧0.8  (7)1.3>f>0.2  (8) where NA is an image-side numerical aperture of theobjective lens, which is required for recording and/or reproducinginformation for the optical information recording medium and f (mm) is afocal length of the objective lens.
 28. The optical pickup device ofclaim 27, wherein the ring-shaped phase structure is a diffractionstructure having a function for diffracting predetermined incident lightand the objective lens forms a converging wave front which is convergedon the information recording surface owing to an effect obtained bycombining a diffraction effect and a refraction effect.
 29. The opticalpickup device of claim 28, wherein the objective lens has a sphericalaberration characteristic that spherical aberration changes in anundercorrected direction when a wavelength of the incident light changesto a longer wavelength.
 30. The optical pickup device of claim 28,wherein when an optical path difference added to a wave fronttransmitted through the diffraction structure is denoted by an opticalpath difference function Φ_(b) defined byΦ_(b) =b ₂ ·h ² +b ₄ ·h ⁴ +b ₆ ·h ⁶+ . . . (wherein b₂, b₄, b₆ . . . are2nd-order, 4th-order, 6th-order . . . optical path difference functioncoefficients, respectively), a following formula is satisfied:−70<(b ₄ ·h _(max) ⁴)/(f·λ ₀·10⁻⁶·(NA·(1−m))⁴)<−20  (8A) wherein λ₀ (nm)is a design wavelength of the objective lens, h_(MAX) is an effectivediameter maximum height (mm) of the optical surface on which thediffraction structure is formed and m is an image formationmagnification of the objective lens.
 31. The optical pickup device ofclaim 27, wherein the ring-shaped phase structure generates thepredetermined optical path difference for the incident light by formingthe adjacent ring surfaces so as to be displaced in an optical axisdirection each other, and the objective lens forms a converging wavefront which is converged on the information recording surface owing to arefraction effect.
 32. The optical pickup device of claim 31, whereinwhen a ring surface including an optical axis is called a central ringsurface, a ring surface adjacent to an outside of the central ringsurface is formed to be displaced in the optical axis direction so as tohave a shorter optical path length than the central ring surface, a ringsurface at a maximum effective diameter position is formed to bedisplaced in the optical axis direction so as to have a longer opticalpath length than an ring surface adjacent to an inside thereof, and aring surface at a position of 75% of a maximum effective diameter isformed to be displaced so as to have a shorter optical path length thana ring surface adjacent to an inside thereof and a ring surface adjacentto an outside thereof.
 33. The optical pickup device of claim 31,wherein a total of the ring surfaces is from 3 to
 20. 34. The opticalpickup device of claim 31, wherein when Δ_(j) (μm) is a step amount ofan arbitrary step of steps in the optical axis direction at a boundaryof mutually adjacent ring surfaces in a ring-shaped phase structureformed in a region from a height of 75% to a height of 100% of aneffective diameter maximum height of the optical surface on which thering-shaped phase structure is formed and n is a refractive index of theobjective lens at a design wavelength of λ₀ (nm), m_(j) represented bym _(j) =INT(X)  (8B) (wherein X=Δ_(j)·(n−1)/(λ₀·10⁻³) and INT(X) is aninteger obtained by half adjust of X) is an integer not less than
 2. 35.The optical pickup device of claim 27, wherein in case that W(λ₀, T₀) isan RMS value of residual aberration of the objective lens when lighthaving a wavelength of λ₀ (nm) which is a design wavelength thereof isincident to the objective lens at an environmental temperature which isa first ambient temperature T₀=25° C., W(λ₁, T₀) is an RMS value ofresidual aberration of the objective lens when light having a wavelengthof λ₁ (nm) which is 5 nm longer than the wavelength of λ₀ is incident tothe objective lens at the environmental temperature which is the firstambient temperature T₀=25° C. and W(λ₂, T₁) is an RMS value of residualaberration of the objective lens when light having a wavelength of λ₂(nm) is incident to the objective lens at the environmental temperaturewhich is a second ambient temperature T₁=55° C., ΔW1 and ΔW2 defined byΔW1=|W(λ₂ , T ₁)−W(λ₀ , T ₀)|  (9)ΔW2=|W(λ₁ , T ₀)−W(λ₀ , T ₀)|  (10) satisfy following formulas:ΔW1<0.035 λrms  (11)ΔW2<0.035 λrms  (12) wherein when λ₀<600 nm, λ₂=λ₀+1.5 (nm) and whenλ₀≧600 nm, λ₂=λ₀+6 (nm).
 36. The optical pickup device of claim 35,wherein the optical pickup device satisfies a following formula:{square root}((ΔW1)²+(ΔW2)²)<0.05 λrms  (13)
 37. The optical pickupdevice of claim 27, wherein the objective lens is an objective lens of afinite conjugate type for converging a diverging light beam emitted fromthe light source on the information recording surface and satisfies afollowing formula:1.1>f>0.2  (13A)
 38. The optical pickup device of claim 37, the opticalpickup device satisfies a following formula:0.2>|m|>0.02  (13B) where m is an image formation magnification of theobjective lens.
 39. The optical pickup device of claim 37, wherein theobjective lens and the light source are united by an actuator at leastto be driven for tracking.
 40. The optical pickup device of claim 21,wherein the optical device satisfies a following formula:0.8<d/f<1.8  (14) where d (mm) is a lens thickness in an optical axis ofthe objective lens and f (mm) is the focal length.
 41. The opticalpickup device of claim 21, wherein the design wavelength of λ₀ (nm) ofthe objective lens satisfies a following formula:500≧λ₀≧350  (15)
 42. The optical pickup device of claim 21, wherein theoptical pickup device satisfies a following formula:0.40≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.63  (16) where X1: adistance (mm) in an optical axis direction between a plane that isperpendicular to an optical axis and tangent to a top of an opticalsurface on a light source side and an optical surface on the lightsource side in a most peripheral portion of an effective diameter(position of the NA on a surface on the light source side to which amarginal light beam is incident), wherein X1 is plus in a case ofmeasuring X1 in a direction of the optical information recording mediumwith reference to the tangent plane, and minus in a case of measuring X1in a direction of the light source, X2: a distance (mm) in an opticalaxis direction between a plane that is perpendicular to an optical axisand tangent to a top of an optical surface on an optical informationrecording medium side and an optical surface on the optical informationrecording medium side in a most peripheral portion of an effectivediameter (position of the NA on a surface on the optical informationrecording medium side to which a marginal light beam is incident),wherein X2 is plus in a case of measuring X2 in a direction of theoptical information recording medium with reference to the tangent planeand minus in a case of measuring X2 in a direction of the light source,N: a refractive index of the objective lens at the design wavelength ofλ₀, f: the focal length (mm) of the objective lens, and m: an imageformation magnification of the objective lens.
 43. An opticalinformation recording/reproducing apparatus comprising optical pickupdevice that comprises: a light source; and a converging optical systemincluding an objective lens for converging a light beam emitted from thelight source to an information recording surface of an opticalinformation recording medium, and is capable of recording and/orreproducing information by converging the light beam emitted from thelight source to the information recording surface of the opticalinformation recording medium with the converging optical system, whereinthe objective lens is a plastic single lens and satisfies followingformulas:NA≧0.8  (1)1.0>f>0.2  (2) where NA is an image-side numerical aperture of theobjective lens, which is required for recording and/or reproducinginformation to the optical information recording medium and f (mm) is afocal length of the objective lens.
 44. The optical informationrecording/reproducing apparatus of claim 43, wherein in case that W(λ₀,T₀) is an RMS value of residual aberration of the objective lens whenlight having a wavelength of λ₀ (nm) which is a design wavelengththereof is incident to the objective lens at an environmentaltemperature which is a first ambient temperature T₀=25° C. and W(λ₀, T₁)is an RMS value of residual aberration of the objective lens when lighthaving the wavelength of λ₀ (nm) which is a design wavelength thereof isincident to the objective lens at the environmental temperature which isa second ambient temperature T₁=55° C., ΔW defined byΔW=|W(λ₀ , T ₁)−W(λ₀ , T ₀)|  (3) satisfies a following formula:ΔW<0.035 λrms  (4)
 45. The optical information recording/reproducingapparatus of claim 43, wherein the design wavelength λ₀ of the opticalobjective lens is not more than 500 nm, and in case that fB(λ₀, T₀) is aback focal length of the objective lens when light having a wavelengthof λ₀ (nm) is incident to the objective lens at an environmentaltemperature which is a first ambient temperature T₀=25° C. and fB(λ₁,T₀) is a back focal length of the objective lens when light having awavelength of λ₁ (nm) which is 5 nm longer than the wavelength of λ₀ isincident to the objective lens at the environmental temperature which isthe first ambient temperature T₀=25° C., ΔfB defined byΔfB=|fB(λ₁ , T ₀)−fB(λ₀ , T ₀)|  (5) satisfies a following formula:ΔfB<0.001 mm  (6)
 46. The optical information recording/reproducingapparatus of claim 43, wherein the objective lens is an objective lensof a finite conjugate type for converging a diverging light beam emittedfrom the light source to the information recording surface of theoptical information recording medium and satisfies a following formula:0.8>f>0.2  (6A)
 47. The optical information recording/reproducingapparatus of claim 46, wherein m satisfies a following formula when m isan image formation magnification of the objective lens:0.2>|m|>0.02  (6B)
 48. The optical information recording/reproducingapparatus of claim 46, wherein the objective lens and the light sourceare united by an actuator at least to be driven for tracking.
 49. Anoptical information recording/reproducing apparatus comprising anoptical pickup device wherein the optical pickup device comprises: alight source; and a converging optical system including an objectivelens for converging a light beam emitted from the light source to aninformation recording surface of an optical information recordingmedium, and the optical pickup device is capable of recording and/orreproducing information by converging the light beam emitted from thelight source to the information recording surface of the opticalinformation recording medium with the converging optical system, whereinthe objective lens is a plastic single lens that comprises a ring-shapedphase structure on at least one optical surface, the ring-shaped phasestructure comprising a plurality of ring surfaces and formed so thatadjacent ring surfaces generate a predetermined optical path differencefor incident light, and satisfies following formulas:NA≧0.8  (7)1.3>f>0.2  (8) where NA is an image-side numerical aperture of theobjective lens, which is required for recording and/or reproducinginformation for the optical information recording medium and f (mm) is afocal length of the objective lens.
 50. The optical informationrecording/reproducing apparatus of claim 49, wherein the ring-shapedphase structure is a diffraction structure having a function fordiffracting predetermined incident light and the objective lens forms aconverging wave front which is converged on the information recordingsurface owing to an effect obtained by combining a diffraction effectand a refraction effect.
 51. The optical informationrecording/reproducing apparatus of claim 50, wherein the objective lenshas a spherical aberration characteristic that spherical aberrationchanges in an undercorrected direction when a wavelength of the incidentlight changes to a longer wavelength.
 52. The optical informationrecording/reproducing apparatus of claim 50, wherein when an opticalpath difference added to a wave front transmitted through thediffraction structure is denoted by an optical path difference functionφ_(b) defined byΦ_(b) =b ₂ ·h ² +b ₄ ·h ⁴ +b ₆ ·h ⁶+ . . . (wherein b₂, b₄, b₆ . . . are2nd-order, 4th-order, 6th-order . . . optical path difference functioncoefficients, respectively), a following formula is satisfied:−70<(b ₄ ·h _(max) ⁴)/(f·λ ₀·10⁻⁶·(NA·(1−m))⁴)<−20  (8A) wherein λ₀ (nm)is a design wavelength of the objective lens, h_(MAX) is an effectivediameter maximum height (mm) of the optical surface on which thediffraction structure is formed and m is an image formationmagnification of the objective lens.
 53. The optical informationrecording/reproducing apparatus of claim 49, wherein the ring-shapedphase structure generates the predetermined optical path difference forthe incident light by forming the adjacent ring surfaces so as to bedisplaced in an optical axis direction each other, and the objectivelens forms a converging wave front which is converged on the informationrecording surface owing to a refraction effect.
 54. The opticalinformation recording/reproducing apparatus of claim 53, wherein when aring surface including an optical axis is called a central ring surface,a ring surface adjacent to an outside of the central ring surface isformed to be displaced in the optical axis direction so as to have ashorter optical path length than the central ring surface, a ringsurface at a maximum effective diameter position is formed to bedisplaced in the optical axis direction so as to have a longer opticalpath length than an ring surface adjacent to an inside thereof, and aring surface at a position of 75% of a maximum effective diameter isformed to be displaced so as to have a shorter optical path length thana ring surface adjacent to an inside thereof and a ring surface adjacentto an outside thereof.
 55. The optical information recording/reproducingapparatus of claim 53, wherein a total of the ring surfaces is from 3 to20.
 56. The optical information recording/reproducing apparatus of claim53, wherein when Δ_(j) (μm) is a step amount of an arbitrary step ofsteps in the optical axis direction at a boundary of mutually adjacentring surfaces in a ring-shaped phase structure formed in a region from aheight of 75% to a height of 100% of an effective diameter maximumheight of the optical surface on which the ring-shaped phase structureis formed and n is a refractive index of the objective lens at a designwavelength of λ₀ (nm), m_(j) represented bym _(j) =INT(X)  (8B) (wherein X=Δ_(j)·(n−1)/(λ₀·10⁻³) and INT(X) is aninteger obtained by half adjust of X) is an integer not less than
 2. 57.The optical information recording/reproducing apparatus of claim 49,wherein in case of W(λ₀, T₀) is an RMS value of residual aberration ofthe objective lens when light having a wavelength of λ₀ (nm) which is adesign wavelength thereof is incident to the objective lens at anenvironmental temperature which is a first ambient temperature T₀=25°C., W(λ₁, T₀) is an RMS value of residual aberration of the objectivelens when light having a wavelength of λ₁ (nm) which is 5 nm longer thanthe wavelength of λ₀ is incident to the objective lens at theenvironmental temperature which is the first ambient temperature T₀=25°C. and W(λ₂, T₁) is an RMS value of residual aberration of the objectivelens when light having a wavelength of λ₂ (nm) is incident to theobjective lens at the environmental temperature which is a secondambient temperature T₁=55° C., ΔW1 and ΔW2 defined byΔW1=|W(λ₂ , T ₁)−W(λ₀ , T ₀)|  (9)ΔW2=|W(λ₁ , T ₀)−W(λ₀ , T ₀)|  (10) satisfy following formulas:ΔW1<0.035 λrms  (11)ΔW2<0.035 λrms  (12) wherein when λ₀<600 nm, λ₂=λ₀+1.5 (nm) and whenλ₀≧600 nm, λ₂=λ₀+6 (nm).
 58. The optical informationrecording/reproducing apparatus of claim 57, wherein the apparatussatisfies a following formula:{square root}((ΔW1)²+(ΔW2)²)<0.05 λrms  (13)
 59. The optical informationrecording/reproducing apparatus of claim 49, wherein the objective lensis an objective lens of a finite conjugate type for converging adiverging light beam emitted from the light source on the informationrecording surface and satisfies a following formula:1.1>f>0.2  (13A)
 60. The optical information recording/reproducingapparatus of claim 59, wherein the apparatus satisfies a followingformula when m is an image formation magnification of the objectivelens:0.2>|m|>0.02  (13B)
 61. The optical information recording/reproducingapparatus of claim 59, wherein the objective lens and the light sourceare united by an actuator at least to be driven for tracking.
 62. Theoptical information recording/reproducing apparatus of claim 43, whereinthe apparatus satisfies a following formula:0.8<d/f<1.8  (14) where d (mm) is a lens thickness in an optical axis ofthe objective lens and f (mm) is the focal length.
 63. The opticalinformation recording/reproducing apparatus of claim 43, wherein thedesign wavelength of λ₀ (nm) of the objective lens satisfies a followingformula:500≧ν₀≧350  (15)
 64. The optical information recording/reproducingapparatus of claim 43, wherein the apparatus satisfies a followingformula:0.40≦(X1−X2)·(N−1)/(NA·f·{square root}(1+|m|))≦0.63  (16) where X1: adistance (mm) in an optical axis direction between a plane that isperpendicular to an optical axis and tangent to a top of an opticalsurface on a light source side and an optical surface on the lightsource side in a most peripheral portion of an effective diameter(position of the NA on a surface on the light source side to which amarginal light beam is incident), wherein X1 is plus in a case ofmeasuring X1 in a direction of the optical information recording mediumwith reference to the tangent plane, and minus in a case of measuring X1in a direction of the light source, X2: a distance (mm) in an opticalaxis direction between a plane that is perpendicular to an optical axisand tangent to a top of an optical surface on an optical informationrecording medium side and an optical surface on the optical informationrecording medium side in a most peripheral portion of an effectivediameter (position of the NA on a surface on the optical informationrecording medium side to which a marginal light beam is incident),wherein X2 is plus in a case of measuring X2 in a direction of theoptical information recording medium with reference to the tangent planeand minus in a case of measuring X2 in a direction of the light source,N: a refractive index of the objective lens at the design wavelength ofλ₀, f: the focal length (mm) of the objective lens, and m: an imageformation magnification of the objective lens.